Fix subpacks.

Signed-off-by: John McCutchan <john@johnmccutchan.com>
Fixed some typos
Fix stack overflow bug. https://github.com/johnmccutchan/asset_pack/issues/6
Adding the ability to pass in a PropertyMapConfig to the PropertyMapImporter. Change fallback to return null,
Merging from master. Removing Expect class code.
Merge branch 'master' into features/video_loader