Proposed: optionally capture active session for tmux

[kaldrenon]

This is a proposed solution for guard/guard#781

By default, when tmux updates the color_location after tests run, it updates the color location for all sessions.

The intention with this code is to only update the color_location for the session that the tests are running in.

[e2]

You simply can pass options[:display_on_all_sessions] as a parameter to the Client constructor - and just add it to all the Client.new() invocations.

Don't hesitate to modify the existing code - it would be better to use something like https://github.com/dominikh/tmux-ruby, and the current code here is only an attempt to refactor the old code.

Ideally, the Client class should be split up into maybe a ClientGroup and Client separately - but that's probably not worth doing. If you have better suggestions let me know.

Also, while I don't care personally, I think it's best to keep the default behavior as the default (showing on all sessions) - just to maybe avoid surprising people.