Allow override of weather-details URL

I HATE HATE HATE the latest Yahoo Weather redesign, so this change allows the user to oveerride (via a preference-tool field) the webpage that's loaded when clicking on the location-name. (I use http://www.weather.com/weather/right-now/10454 to get weather.com's far superior local weather page for my area.)

If another website is preferred over the Yahoo Weather
details page normally loaded when clicking on the location
name, it can be configured in the preferences. The URL
provided is used exactly as-is, so any embedding of location
details must already be present.