Add support to prefix the key with tmux support

- Add the --tmux command line option
- Prefix the key in the authorized keys file with a command that
  immediately directs the pair to the tmux session when the option is added
- Remove tmux prefix and the key when the key is removed

This feature is based on the pairing setup here: http://www.zeespencer.com/articles/building-a-remote-pairing-setup/. I got tired of pasting the tmux prefix manually, so I added a command line option to add it for me.

Despite the spirt of pairing, I hackety-hacked this solo, so please let me know if you have any suggestions/improvements. Thanks!