Allow tmux notifier to receive custom types.

This allows the tmux notifier to receive notifications types like
:starting (for instance, when the tests start running).

It will work with the same precedence rules as the already known types.

See issue #488 for my previous comments on why this could be interesting.
Line is too long. [99/80]<br>Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.