add ping/pong response to ping.py

[RhinosF1]

The problem

The default ping module doesn't include ping/pong despite it's name and I'd rather not replace it just for that (I could move it out of core hacks but why not upstream)!

The solution

Add it, I will after I write this.

Example

09:54:36 <+RhinosF1> .ping
09:54:37 <@ZppixBot-test> Pong!