Allow custom debug logging

[jirenius]

Issue

In order to resolved issues like #39, it should be possible to capture logging using a custom function.
This will extend the current option for debug from:

* @param {boolean} [opt.debug] Flag to debug log all WebSocket communication, or logging function. Defaults to false.

To:

* @param {boolean|function} [opt.debug] Flag to debug log all WebSocket communication, or logging function. Defaults to false. Function has schema: (type:string,msg:string,ctx:any) => void.