tmux sessions support

Adds support for listing number of detached tmux sessions.

Adds up number of detached screens sessions and number of detached tmux sessions as a total of detached sessions. Most users only use one or the other (I would hope so at least), so adding them up is more efficient use of space in the prompt than listing them separately.

This addresses the "FIXME: add tmux support".

Also updates README about tmux support.