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 This here means that number of args that this switch takes is one or more after the `--` (it doesn't include the `--metadata` itself) but setting it to "1" blows up the parser if you omit the parameter. It's not perfect but it't the best way to handle the edge cases.