Support authentication through Github Enterprise

[csabakoncz]

config.yml is consulted for various Github URLs so that Jingo can connect to Github instances hosted at arbitrary locations.

E.g. to login using https://mygithub.com one can add the following configuration (tested with Github Enterprise 2.6):

github: enabled: true redirectURL: '' clientId: clientSecret: authorizationURL: https://mygithub.com/login/oauth/authorize tokenURL: https://mygithub.com/login/oauth/access_token userProfileURL: https://mygithub.com/api/v3/user