Add a way to hint implementation-files to --watch

[ELLIOTTCABLE]

Currently, --watch only re-runs your tests when the test files themselves change. Obviously, that's somewhat less than ideal.

I'd love to see --watch take an optional argument, that being a glob of files or directories (other than the tests, which are already listed at the command-line) to subscribe to changes for.

Example:

mocha --watch Source/ --reporter dot --watch Test/*.js

[jbnicolai]

Closing in favor of #1780