add: Stat polling support while `fs.watch` doesn't work.

See issue: https://github.com/jashkenas/coffee-script/issues/3210

Add a new cli option: -g --polling [SPAN]

    If state polling mode is enabled, use it.
    Else use the native api.

    This is useful while watching remote directory.
    Such as the `fs.watch` won't catch the SMB server's file change event.