add Spotify auth provider

Author: thebradbain

lib/authProviders.js

@@ -73,7 +73,18 @@ export const authProviders = {
       client_id: [notEmpty],
       client_secret: [notEmpty]
     })
-  }
+  },
+  'spotify': {
+    auth_version: "2.0",
+    authorize_url: 'https://accounts.spotify.com/authorize',
+    api_url: 'https://api.spotify.com/',
+    callback_url: ({app_name}) => `${app_name}://authorize`,
+
+    validate: validate({
+      client_id: [notEmpty],
+      client_secret: [notEmpty]
+    })
+  },
 }
 
-export default authProviders;
+export default authProviders;