Add StubPlugin::load_plugin()

The class method returns the instance of the loaded plugin
instead of its class like StubPlugin::new_plugin()
Make conf in plugin configurable

Add []= and [] methods to ConfStub instances
Add update_weather_url

We need to update the NOAA weather source URL from
http://weather.noaa.gov/weather/current/#{station_id}.html
to
http://www.aviationweather.gov/adds/metars/?station_ids=#{station_id}&std_trans=translated&chk_metars=on&hoursStr=most+recent+only