Added a way for handle_action() to accept "speak"

[takluyver]

I'm going to close this for now, in favour of tiles catching 'speak' themselves. Some examples of the complexities:

• There might be more than one character on a tile, so the user needs to speak to Fred.
• The user might need to say specific things: for example there's already a tile (e2) with a little number-guessing game.

So overall, I think it's easier to handle speak like other actions inside the tile. We can always revisit this later.