tbtool: Add 'shell' command, to go straight to a bash shell on Tingbot

[joerick]

This will be especially compelling when we've implemented saving a Tingbot hostname, it'll be tbtool shell to get a shell.

[joerick]

Will need a little work to plumb together stdin/stdout to Paramiko

[joerick]

http://docs.paramiko.org/en/2.0/api/client.html#paramiko.client.SSHClient.invoke_shell

[joerick]

Windows doesn't have select(), which makes this a lot harder. Threads are probably the way to go.

I've got a start of this in a branch, if anyone wants to look at it see here: https://github.com/tingbot/tingbot-python/tree/paramiko-shell