Load settings from a configuration file

This commit allows settings to be loaded from a configuration
file. If one exists at `$XDG_CONFIG_HOME/screenplain/screenplain.cfg`
(by default), it will be loaded.

A new `-c`/`--config` command line flag allows loading a configuration
file explicitly. Command line options take precedence over the settings
loaded from the configuration file.

The format of the file is INI, with no interpolations, options without
values are allowed, and double-bracketed "sub-sections" (aesthetics
only, the hierarchy of such sections in relation with "root-level"
sections is not enforced).

An example of a configuration file would be:

```ini
    [export]
        format: pdf

        [[pdf]]
            strong: no

        [[html]]
            base: no
            css
```

screenplain/config.py

@@ -0,0 +1,61 @@
+import os
+import re
+import json
+import configparser
+
+
+class Defaults:
+    FILENAME_CONFIG = 'screenplain.cfg'
+    PATH_CONFIG = os.path.join(
+                      os.getenv('XDG_CONFIG_HOME')
+                          or os.path.join(os.getenv('HOME'), '.config'),
+                      'screenplain',
+                      FILENAME_CONFIG,
+                  )
+
+
+class ConfigurationFileError(Exception): pass
+
+
+class ConfigurationFile(configparser.ConfigParser):
+    def __init__(self, path=Defaults.PATH_CONFIG):
+        super().__init__(interpolation=None,
+                         allow_no_value=True,
+                         converters={
+                             'list': self.__getlist,
+                         })
+
+        # Allow brackets in section names
+        self.SECTCRE = re.compile(r'^[ \t\r\f\v]*\[(?P<header>.+?)\][ \t\r\f\v]*$')
+
+        # Initialize sections and their expected values
+        self.read_string("""
+            [export]
+                format
+
+                [[pdf]]
+                    strong: no
+                    font
+
+                [[html]]
+                    base: no
+                    css
+
+            [font]
+        """)
+
+        try:
+            self.read(path)
+        except configparser.Error as e:
+            raise ConfigurationFileError('unable to load configuration file: %s' % e)
+
+    def __getlist(self, v):
+        try:
+            v = json.loads(v)
+        except json.JSONDecodeError as e:
+            raise ConfigurationFileError('unable to decode JSON value: %s' % e)
+
+        if not isinstance(v, list):
+            raise ConfigurationFileError('value is not a list: %s' % type(v))
+
+        return v

screenplain/main.py

@@ -4,11 +4,13 @@
 # Licensed under the MIT license:
 # http://www.opensource.org/licenses/mit-license.php
 
+import os
 import sys
 import codecs
 from optparse import OptionParser
 
 from screenplain.parsers import fountain
+from screenplain.config import ConfigurationFile, ConfigurationFileError
 
 output_formats = (
     'fdx', 'html', 'pdf'
@@ -32,6 +34,14 @@ def invalid_format(parser, message):
 
 def main(args):
     parser = OptionParser(usage=usage)
+    parser.add_option(
+        '-c', '--config',
+        metavar='CONFIG',
+        help=(
+            'Path to the configuration file to load (superseeded by command '
+            'line options)'
+        )
+    )
     parser.add_option(
         '-f', '--format', dest='output_format',
         metavar='FORMAT',
@@ -43,6 +53,7 @@ def main(args):
     parser.add_option(
         '--bare',
         action='store_true',
+        default=False,
         dest='bare',
         help=(
             'For HTML output, only output the actual screenplay, '
@@ -60,6 +71,7 @@ def main(args):
     parser.add_option(
         '--strong',
         action='store_true',
+        default=False,
         dest='strong',
         help=(
             'For PDF output, scene headings will appear '
@@ -72,8 +84,27 @@ def main(args):
     input_file = (len(args) > 0 and args[0] != '-') and args[0] or None
     output_file = (len(args) > 1 and args[1] != '-') and args[1] or None
 
-    format = options.output_format
-    if format is None and output_file:
+    try:
+        if options.config:
+            if not os.path.isfile(options.config):
+                sys.stderr.write('no such file: %s' % options.config)
+                return
+            config = ConfigurationFile(options.config)
+        else:
+            config = ConfigurationFile()
+    except ConfigurationFileError as e:
+        sys.stderr.write('error: %s' % e)
+        return
+
+    if options.output_format:
+        config['export']['format'] = options.output_format
+    if options.css:
+        config['[html]']['css'] = options.css
+    config['[html]']['bare'] = str(options.bare)
+    config['[pdf]']['strong'] = str(options.strong)
+
+    format = config['export']['format']
+    if not format and output_file:
         if output_file.endswith('.fdx'):
             format = 'fdx'
         elif output_file.endswith('.html'):
@@ -121,14 +152,16 @@ def main(args):
             from screenplain.export.fdx import to_fdx
             to_fdx(screenplay, output)
         elif format == 'html':
+            html_options = config['[html]']
             from screenplain.export.html import convert
             convert(
                 screenplay, output,
-                css_file=options.css, bare=options.bare
+                css_file=html_options['css'], bare=html_options.getboolean('bare')
             )
         elif format == 'pdf':
+            pdf_options = config['[pdf]']
             from screenplain.export.pdf import to_pdf
-            to_pdf(screenplay, output, is_strong=options.strong)
+            to_pdf(config, screenplay, output, is_strong=pdf_options.getboolean('strong'))
     finally:
         if output_file:
             output.close()