AppUpdater: Add ability to include metadata files in diffs

Branched from https://github.com/endlessm/xdelta3-dir-patcher/pull/11 so don't merge until that PR is included.

[endlessm/eos-shell#3985]
@dbnicholson I see your point here.The problem is that the parser halts all execution if params aren't correct so in some scripts we've had problems when arg variables weren't declared or were empty strings (I can't remember theoriginal problem since the history got mangled when these scripts were moved into this repo folder) so we started sprinkling these around to make them more robust at the price of allowing some weird use cases. Maybe we can start going back to the proper way of doing it and see if anything breaks again.