Add toggle pause key command

Sometimes, the user may want to pause the file scanning (maybe, he wants to modify several files and doesn't want rerun to keep restarting as each is saved).  This feature adds a pause toggle key command (press 'p') which pauses and un-pauses the file watcher.

* Add call to new Watcher functions pause and unpause
* Add call to Listener#pause and Listener#unpause
* Add spec for testing pause toggle