Upgrade Commoner to --watch directories instead of individual files

This behavior is new in Commoner v0.8.3, following the incorporation of @jeffreylin's `DirWatcher` implementation:
https://github.com/jeffreylin/jsx_transformer_fun/blob/master/dirWatcher.js

Watching directories instead of files reduces the total number of open files, and copes better with editors that save files by deleting and then immediately recreating them.

Closes #60.
Closes #71.

cc @jeffreylin @zpao @petehunt