Add override for weather-details URL

I **HATE** the new Yahoo Weather webpage. (The one loaded when clicking on the city name.) I hated the old one, but I hate the new one enough to actually _do something about it_.

While I'm not crazy about all of the changes weather.com have made recently, I'd still much rather get my weather information from them.

So, in the spirit of the 'city' configuration setting, this changeset (based off of the gnome3.4 branch) adds an optional config 'details-url'. If set to a URL (static string, nothing fancy or generated... so the location needs to be embedded, and manually updated if it changes), then clicking on the city name in the weather popup will load the specified page instead of the Yahoo Weather awfulness. (As with 'city', blanking the 'details-url' config resets the link to Yahoo's default.)

I now have mine set to `http://www.weather.com/weather/today/10454`, and all is right with the world. :-)

(Note: This is my first foray into github's fork/pull-request universe — I've just submitted patches in the past. So please let me know if I've horked anything along the way, and I'll try to course-correct.)