Make lunchbot remember where lunch is

Now lunchbot will remember where the latest selection for lunch is when someone
tries to ask for lunch a second time in the same day.  Lunchbot will hold onto
the currently selected lunch choice for four hours (hard-coded, non-configurable
... for now).

The first request for "!lunch" will result in:

  lunchbot says: <lunch-spot>

All subsequent requests for "!lunch" will result in:

  lunch is at <lunch-spot>; sorry if you don't like it, blame <user>