Support indented heredocs

I initially attempted to implement this with (regex) conditional
expressions after seeing an apparent backreference in an end
pattern referencing a capture group from a begin pattern. I soon
learned that TextMate performs a string replace on the end
pattern first (replacing things that look like backreferences with
the captured values from the begin patterns) before the end
pattern is actually used as a regex. TextMate does not however do
any such magic that would support conditional expressions in end
patterns referencing capture groups from begin patterns.

Infininight recommended some regex magic that works within the
existing framework of the TextMate grammar though, and this magic
enables us to make each existing heredoc rule work for both
regular and indented heredocs, instead of duplicating each rule
and maintaining one set of rules for regular heredocs and another
set of rules for indented heredocs.

So what is the magic? Given an end pattern of
(^((?!\1)\s+)?HTML$), \1 is replaced by TextMate before being
used as a regex with the contents of capture group 1 from the
begin pattern (as mentioned above). Imagine that the begin
pattern is <<(~)HTML, and so \1 indicates whether or not the
heredoc being matched is an indented heredoc.

In the event the heredoc is indented, we have (^((?!~)\s+)?HTML$).
The negative look-ahead for ~ will always match a correctly-
formatted terminator, thereby enabling the presence of \s+
preceding the terminator. The ? following the whole ((?!~)\s+)
means that the preceding whitespace isn't _required_ when an
indented heredoc is used.

In the event the heredoc is not indented, we have
(^((?!)\s+)?HTML$). The negative look-ahead with no contents
never matches, and thus the \s+ will never be part of a match,
meaning whitespace will never be allowed for non-intended heredocs.

It's magic.