Turn into a library

[vimalloc]

Make the relay portion of this a library, where you can register callback handlers for incoming data. This is totally unnecessary for the app I'm building, but I think it would have a few benefits for learning rust, namely

1. How to build and use a library.
2. Dealing with threading so users can send messages independently of recving them

[vimalloc]

Need a way to register callback functions. Without knowing anything about how rust could acomplish this, my first thought is storing functions in a key/value mapping. Every function will need to have a signature like:

fn some_callback(hdata: HData, Option<<some_generic_box_pointer>>)

The hdata is obvious, that is the contents of the message. The generic box pointer then is less so. If they have a callback method, they may need some additional state to handle it. In the case of playing a sound on highlight, I wouldn't need it, so Option would be None. But if they were wanting to keep track of, say, who you have unread messages from, or really anything more complex then playing a sound, they will probably need some external state. Passing it back to the function call seems better then making them use a global somewhere. Although, maybe there is a better way to do all this, I'm just thinking here. Maybe something with traits could help? Dunno.