Support custom OAuth servers in config

[aaronpk]

I'd love to be able to change the OAuth server that this uses to a custom server. Given that it's using the generic OAuth client, it looks like you'd just need to add a few settings to the config.yml file.

The OAuth client [1] seems to take the following config values: ClientID, ClientSecret, Scopes and Endpoint which contains both the authorization and token endpoints. [2] Moving these to the config file should make it super easy to use with any OAuth 2 server!

[bnfinet]

implemented at 57e83cf