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
```

This commit allows the configuration file to declare custom fonts to
be used, when exporting a PDF file.

The functionality could probably be extended to HTML exports.

Example that sets "Courier Prime" as default font:

```ini
[export]
    format: pdf

    [[pdf]]
        strong: no
        font: courier_prime

[font]

    [[courier_prime]]
        name: Courier Prime
        regular: ["Courier Prime", "courier-prime.ttf"]
        bold: ["Courier Prime Bold", "courier-prime-bold.ttf"]
        italic: ["Courier Prime Italic", "courier-prime-italic.ttf"]
        bold_italic: ["Courier Prime Bold Italic", "courier-prime-bold-italic.ttf"]
```