Multi-seat support

[nsensfel]

Not to be added in the first version, and only possibly as an extension afterwards, but allowing narrative with multiple users would be a cool addition.

I guess it's pretty much the equivalent of multi-threading support:

• User specific variables/registers.
• Semaphores.
• User specific program counter.
• Fork/Join for any given subset of users.
• User specific output & choices.

[nsensfel]

Much more basic multi-user support, an extension that adds a (get_username) computation returning a string that corresponds to the user that made the last input (prompt or choice). Combined with the dict structure (#25), this should allow fairly simple multiplayer.
Some timer extension that generates events after a given time would be a nice addition for multiplayer, to detect when a player takes too long to act.