Create a default tmux session only when none defined in tmux.conf

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>
Support $PYTHONUSERBASE per PEP 370

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>
Merge branch 'master' of github.com:sorin-ionescu/prezto

Conflicts:
	modules/python/init.zsh
	modules/tmux/init.zsh