Async Pong support

The idea is to cause a CursorHold without waiting updatetime milliseconds. This way vim can be notified earlier when something happens.

Currently there are only two implementations.

- TTY: This [tiny patch](https://gist.github.com/mustrumr/9489318) makes it possible to map keys to CursorHold. Map \uFEFF to &lt;CursorHold&gt;. Inject \uFEFF in the tty.
- gtk+: Create a pipe and listen. doautocmd CursorHold when readable.