Add show_dialog auth url option

Optional. Whether or not to force the user to approve the app again if they’ve already done so. If false (default), a user who has already approved the application may be automatically redirected to the URI specified by redirect_uri. If true, the user will not be automatically redirected and will have to approve the app again.

From https://developer.spotify.com/web-api/authorization-guide/

spotipy/oauth2.py

@@ -163,7 +163,7 @@ class SpotifyOAuth(object):
     def is_token_expired(self, token_info):
         return is_token_expired(token_info)
 
-    def get_authorize_url(self, state=None):
+    def get_authorize_url(self, state=None, show_dialog=False):
         """ Gets the URL to use to authorize this app
         """
         payload = {'client_id': self.client_id,
@@ -175,6 +175,8 @@ class SpotifyOAuth(object):
             state = self.state
         if state is not None:
             payload['state'] = state
+        if show_dialog:
+            payload['show_dialog'] = True
 
         urlparams = urllibparse.urlencode(payload)