add do-not-watch configuration options

There are some cases where it is desirable to never watch a specific directory (https://github.com/facebook/buck/issues/616)

I haven't thought a huge amount about this, but there are two ways that come to mind:
- add a `.watchmanconfig` option that prevents watching a project tree (simplest for unprivileged users)
- add `/etc/watchman.json` option that prevents watches a list of paths (more suitable for administrators)

Behavior would be similar to https://facebook.github.io/watchman/docs/config.html#illegal_fstypes