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 remote-tracking branch 'upstream/master'

Update last commits from upstream: remember check that the prompt module's new
position is not wrong. No other collisions seem to have happened...