Allow users to specify ignored directories for the -w option or ignore all hidden directories

[brodo]

Currently 'node_modules' and '.git' are excluded per default. It would be nice if the user could add more ignore paths. Alternatively, maybe all hidden directories should be ignored per default.

[tj]

gets too complicated, --watch needs to be completely thrown out and put into a stand-alone lib so jade/coffeescript/stylus/mocha/everything-else can stop re-inventing

[jareware]

Something that --watch currently has going for it is speed. Especially if you have e.g. Babel doing transpilation, invoking mocha separately for each test run is immensely slower than the re-run watch mode does.

Ideas on how to work around this with an external watcher?