[Feature Request] Support for OpenWeather

Years ago in the old bugtracker there was an issue for the usage of openweathermap.org. The TODO file states since five years:

* While met.no is a good forecast provider, it might be better to use
  another free and non-commercial provider for the current weather.
  Viable candidates:
  - openweathermap.org: could be developed a bit more, so check it
    again in a year or so

The openweathermap.org API is also capable of producing XML responses, I've attached one example.

test.xml