Add support for shallow directory watching

[natebosch]

For scenarios where you want to recursively watch a directory except for a list of subdirectories we'd like to be able to set up a shallow watcher at the root and then recursive watchers on all subdirectories except the ignored ones. It should be relatively straightforward to implement if we have a shallow directory watcher.

We could also put the ignored subdirectory behavior directly in this package.

We'll need to confirm that all the OSes support a shallow watch.