Prompt for wait when livereload alreading running

``` sh
GOBBLE INFO    server listening on port 4567
GOBBLE WARNING a livereload server is already running (perhaps in a separate gobble process?). Livereload will not be available for this session
```

When this happens, I always CTRL+C, go stop the other server and restart gobble. I think a prompt for `retry/continue anyway` would be better  
