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]
Are you sure you want `nargs='?'` here? That says you can have 0 or 1 arguments. I can't see how specifying `--metadata` with no argument is useful. If it's not specified, it will get your default `None`.