Add a listener where pre-sending logic can be enforced.

[camclendenin]

Right now, I don't see an easy way to enforce a character limit on outgoing messages, or to even handle situations such as detecting an issue with network connectivity. There are a number of kinds of checks I'd like to be able to do before the message is "sent" and displayed in the UI. This could be called something like setShouldSendMessageListener for example.