Possibility to define redirectURL in the config

[dangrahn]

There should to be a possibility to define the redirectURL in the config, since some oauth providers only accepts one single pre-defined redirect url. This means that it won't be possible to use e.g. the 'http://www.facebook.com/robots.txt' one if you add oauth authentication on e.g. your main site as well (hence need another redirectURL than facebooks robot.txt).

I tried to add this support, but ended up getting problems after the redirect since the config wasn't known at that point (since it hadn't been saved to local storage yet) hence I couldn't call the redirectURL(config) since config was unknown. I'm sure this can be solved but I haven't had time to go to the bottom with this, so right now I just modified the redirect url in the provider library file.

Btw, thanks for a great extension oauth library! Have made my life a lot easier already! :)