Allow specifying callbacks for command events

[pmeinhardt]

As pointed out by @rjanja in #52, being able to receive "notifications" about various events (exec, receive, …) during the execution of a command will be needed by applications built on top of SSHKit.

We should therefore:

1. identify which events an application would want to receive a notification
2. define how to pass (optional) callback functions to SSHKit.run/2, SSHKit.run/3, e.g.
   • pass separate functions for each event, or…
   • a single function that receives an event type + event data, or…
   • or pass a callback module implementing a defined interface?
3. specify what information should be passed to the callbacks (likely host, command info…)

Let's gather some thoughts here and figure this out 🙂

[pmeinhardt]

Sending lifecycle/status messages to the process might be another idea, similar to:
https://github.com/edgurgel/httpoison#async-requests

Need to think about this a little more though.

[pmeinhardt]

I think this should be addressed by the new streaming interface. The stream will emit all the channel messages and these messages allow you to access the channel, connection and host information. ✌️