Previously, it was not possible to set an arbitrary tmux prefix since
CTRL was hardcoded in the module.
To avoid breaking existing configs, a new option was implemented that
conveniently uses the tmux terminology but defaults to null and does
not affect previous behavior when set to null.
The behavior for the shortcut option was not completely replicated,
i.e., it does not bind "b" to send-prefix but stick to the default of
the prefix binding sending prefix (C-b C-b instead of C-b b) and it
does not bind repetition of the prefix (C-b C-b) to `last-window`,
both of these bring the option closer to the default tmux
configuration.
Fixes#1237
