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>
Customizations to zprezto (and a new prompt)
Bind CTRL+R to incremental history backward search