Filter Songkick results based on location

The Songkick API key was not stored in config.ini, and the default value was stored in __init__.py
I removed it from there and it's now saving properly in config.ini

I added an optional configuration to filter per location.
So far it's only working with a numerical metro-area id.

TODO:
- I would like to change the Songkick configuration section to make it completely optional.
- I need to dig a bit on the API key topic for songkick, not sure it should have a default one (same as all the notification services) like what @maiis did in his pull request.
- I would need to add some kind of search button to find a location using actual words