Enable load-session command from outside of Tmux

This means that "init" session layouts can now be used to start and/or
re-attach to your main session.

Personally I have a "main" session layout with a couple of default
windows. I can now either start that session, or re-attach to it from
outside of Tmux by just running `tmuxifier s main`, instead of manually
creating a session named "main" and then loading my windows into it.
Bump version to 0.5.0