title: Adding external providers to sorcery
body: <p>I have an app with authentication, which has been done with devise and used omniauth to sign in from various providers (including vk.com, google, twitter and so on). Everything seemed working well, except that I needed to include a separate gem for every provider.</p>
  <p>Now I decided to make that authentication much lighter and add some logging, so I decided to move from devise to sorcery. But now I have a problem with external providers, 'cause the gem has only few (no vk.com or yahoo, which I do need).</p>
  <p>I tried to understand the code of the externalproviders to add my own just alike, but it seems a little bit too complicated for me. Is there any easier way around?</p>