Add new tmux mode

[jubeaz]

Hello,

here is a PR that will solve this issues : #57

I've added a new option for a new way to manage tmux. I've added documentation on the way to use it in README.md

I did not touch to the old one in case you want to keep it but for me it was not working properly because if there are several tmux sessions running there are no guaranty that the command will be sent to the proper session / window.

I've used the same pattern to import tmux lib but I think it could be included globally ?

I did not provide prefil_shell_cmd fall back since I'll raise an exception before launching the gui if libtmux can't be loaded or if session does not exist (check_tmux function)

[jubeaz]

Don't know what to do with that