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.
Merge pull request #150 from benjamn/commoner-dir-watcher

Upgrade Commoner to --watch directories instead of individual files