Allow override of weather-details URL. Incorpore the changes from

https://github.com/canek-pelaez/gnome-shell-extension-weather/pull/12

but don't expose the option in the preferences dialog.
README: Add instructions to override the weather detail URL. This,
along with commit ba7d523c5d048ab9183bd65a37631e46d329a2a1, fixes
issue 12:

https://github.com/canek-pelaez/gnome-shell-extension-weather/pull/12