Add ability to detect files not included in the config

[scalvert]

Our app uses a TON of engines and in-repo addons. It would be great to detect when the build had files that weren't present in the config. Conversely, it would be great to also detect if there were config entries present that were not represented by real files.

It could be possible to implement this using fs-tree-diff, which would allow us to have human readable output indicating whether something was missing on one side or the other.