Add a method on Messages to extend messages commands.

[braydonf]

Example usage:

var p2p = require('bitcore-p2p');
var Message = p2p.Message;
function CustomMessage(arg, options) {
  Message.call(this, options);
  ...
}
inherits(CustomMessage, Message);

var messages = new Messages();
messages.extend('command', 'MessageName', CustomMessage);

[braydonf]

Included in: https://github.com/bitpay/bitcore-p2p/commit/d4958eb05fd02f0a597e18ad5ca56b4a051638c6