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
Small readme update
Re-organize internal files a bit
Re-organize and improve help command and help info for each command
Change output format of list commands
Bump version to 0.5.1
Misc. code cleanup and minor improvement to help command
Update aliases and fixed inconsistencies with list command aliases
Bump version to 0.6.0
Merge branch 'master' into new-init-system