Feature request: Tunnels can override their return point

[joningold]

New syntax:

->-> go_here_instead

The knot-name following the ->-> is an "override" to the return. This causes the game to return from the tunnel, but then immediately divert away to go_here_instead, instead of continuing the flow as normal.

Use case: The following option might appear in several locations of the game:

* [Go to sleep] -> sleeping ->

But sleeping might have the following code:

=== sleeping == 
 { shuffle:
     - You sleep, and then wake. ->->
     - You sleep, but wake to find yourself somewhere new.... 
        ->-> woken_up_in_ 528_AD 
 }

...which allows a sleeping player to be surprise-teleported.

[joethephish]

Included in 311aef7.