add username/password authentication.

hey!

first of all, thanks for sondcloud2000! :+1: 
i implemented the feature mentioned in https://github.com/grobie/soundcloud2000/issues/16 but i'm not sure what to do with it, maybe someone can take this further.

also i'm not sure how to handle the client_secret, do you have any ideas?

this is how it works:
first auth: start the app with --auth option and  provide your username and password
once this is done, the access token gets refreshed every time you start the app

any feedback is welcome! 
ben