[extractor/common] Add method for extracting form hidden input fields as dict

youtube_dl/extractor/common.py

@@ -705,6 +705,12 @@ class InfoExtractor(object):
         return self._html_search_meta('twitter:player', html,
                                       'twitter card player')
 
+    @staticmethod
+    def _form_hidden_inputs(html):
+        return dict(re.findall(
+            r'<input\s+type="hidden"\s+name="([^"]+)"\s+(?:id="[^"]+"\s+)?value="([^"]*)"',
+            html))
+
     def _sort_formats(self, formats, field_preference=None):
         if not formats:
             raise ExtractorError('No video formats found')