Feature (CLI): Dependency checker.

[ryan-roemer]

Should be pretty straightforward to add a check-deps CLI action that takes:

1. An input file (or files) (or all files in repo not in node_modules or gitignored)
   2.A package.json

... and then outputs:

1. Unused packages in package.json
2. Missing imports

See https://www.npmjs.com/package/depcheck for other ideas...