Add scrobble_filter to provide a configurable list of URI schemes not…

[fatg3erman]

… not scrobble

This should fix #28, however I've been testing it today and found that even with the Spotify web app scrobbling enabled, I don't see double scrobbles from Mopidy even if mopidy is also scrobbling.

Not sure why this is, perhaps it's only temporary, but adding this as a config value dosn't do any harm.

[fatg3erman]

The failure appears to be something in pylast that I haven't changed, so I don't know what's failing. It runs on my system.

[jjok]

Looks good. I think I would call it "blacklist" though, to make it clearer that those extensions won't be scrobbled. It also means that you can add a "whitelist" at some point too.

Maybe backend_blacklist or extension_blacklist?

[fatg3erman]

I agree. backend_blacklist is better. It's still failing the checks though but I don't know why. It's running fine for me.