support for OAuth2 custom support.

[samifruit514]

Hello,

We would like to use our private Oauth2 provider. After looking at the code, it doesn't seem too complicated to add a feature to support custom providers as most of the providers in package "golang.org/x/oauth2" only have an auth url and token url.

Do you think it would be a good idea to add such feature?

Thank you

[prydonius]

Hey! Yeah I did want to make this customisable at some point. If you'd like to give that a go, that would be great!