The cache time is now configurable by the user.  If no cache time is
set, the old default of half an hour is still used.

Temperature formatting has been factored out into its own function.

It has the same functionality as my old shell script, but now it's
written in Python.  The code is far from the most elegant it could be,
but it's a decent example of how the new plugin API can be used for
something practical.