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>
make tmux module have optional attach behavior

This commit reverts the tmux module back to the original behavior but
doesn't set `destroy-unattached` to on `on` globally and provides an
option for attaching to the background or last-used session.