Create a better mod/patch file format

[apocalyptech]

There's a number of things we'd like to do in BLCMM which would require updates to the mod/patch file format (some examples: issues #9, #14, #16, and #17). For most of those, the changes required to support it are pretty minimal, but there's one other complicating factor: the current .blcm format isn't the best, and rather than continue to patch it up over time, it might make more sense to just move to something better.

The current format sort of looks like XML but deviates from XML in many ways. This was intentional at the time -- The Java-based XML parsers that LightChaosman had tried out were prohibitively slow and memory-hungry at the time. Bigger mods would become essentially unmanageable. So, a lighter-weight XMLish format was created instead, with custom parsing which could be a bit quicker. Those concerns might not apply anymore today, and it's possible that there's faster XML-parsing engines out there which could be used instead, even if the "default" ones are still slow.

However, I don't think anyone's really fond of XML anyway, so if we're gonna do a full revamp, I'd probably prefer doing something totally different anyway. apple1417's started a gist with some more background and some preliminary suggestions, which as-proposed there would be YAML-based. That might be a decent place to start, and at any rate we'd want to make sure to coordinate this with unrealsdk development and Command Extensions, so that we can ensure things continue to work all right.