Get list of updated files for extension

[tmaier]

It would be great if an extension could get a list of the files updated on after_build

Related:

• http://forum.middlemanapp.com/t/get-a-list-of-created-and-updated-files-in-extension/1368/2
• Only invalidate updated files andrusha/middleman-cloudfront#10

/cc @manuelmeurer

[tdreyno]

This seems like a sensible feature. Right now, the build step is tied directly into Thor. We'd need to abstract that a bit to provide this additional information. This abstraction is a "nice to have" for v4. We'd definitely welcome PRs or guidance on how exactly such a feature should work.

[tdreyno]

Thanks for the inspiration. We're not tracking this feature in #1319