Get/set/display seed

[matt-dray]

Generate a seed that can be provided back to the user somehow, e.g. printed as part of the UI (bit annoying)or the start message (once it's gone it's gone); maybe <<- to put it in the user's environment; or possibly it's the only thing that actually get returned from start_game()?

Allow the user to provide a seed to start_game() so they can recreate a dungeon.