Add /etc/weather/weatherrc to config search.

* weather.1(INPUT FILES): Updated the list of potential weatherrc
locations to reflect those mentioned in the INSTALL file, particularly
the addition of /etc/weather/weatherrc in the 2.0 release.

* weather.py(get_config): Adjusted the configuration search locations to
include /etc/weather/weatherrc, since the INSTALL file started
mentioning it in the 2.0 release even though it wasn't actually
implemented as pointed out by Ben Kohler.