Implements pipe_register()

[shikokuchuo]

Registers a callback to be run upon {add, remove} pipe events at a Socket.

@wch pls see if this meets your needs or you prefer anything to be tweaked.

Closes #114.

[shikokuchuo]

This is very likely going to be useful for what we need, so I'm going ahead to merge this.

For reference, this registers the callback directly rather than with a 'promise' as it's triggered on a pipe event (connect, disconnect), and there's no object associated with it, unlike for example an 'aio', which can be turned into a 'promise'.