Conditional fall back

This request is sort of crazy but would also be quite useful I believe. What I'm thinking is that it would be awesome to have a way to say that the build should fall back to rebuilding the image but only when part of a file has been changed.

I'm currently doing some node.js stuff and the package.json file is... well, quite abused as per usual. Most changes in there don't affect the running application at all, except for the dependencies and devDependencies sections. Everything else is basically just developer configuration but it still has to rebuild everything just in case I did change any dependencies. If I could tell Tilt either to parse the JSON and check where the differences are or if I could specify some regex for where to look, that would speed up iteration times quite a bit for me.

Start by reviewing Tilt's existing image build and file-change handling; the issue does not name specific files, tests, or entry points. Clarify how changes within package.json should be detected, especially dependencies and devDependencies, and define done as rebuilding only when those sections change.