WIP: Allow wemux usage. #88

The goal here is to allow the use of wemux with tmuxinator. Wemux is not
a complete wrapper of tmux, so it presents some interesting challenges.
So far this code is able to generate windows and panes for wemux given a
standard template with a tmux_command of wemux, though the commands sent
to those panes seem to be stripping all spaces. There are also a couple
of failing tests, which I have not been able to dig into yet, but I plan
to.