Plugin support

Extractor plugins are loaded from <root-dir>/ytdlp_plugins/extractor/__init__.py

Inspired by https://github.com/un-def/dl-plus

:ci skip dl

ytdlp_plugins/extractor/sample.py

@@ -0,0 +1,12 @@
+from __future__ import unicode_literals
+
+from youtube_dlc.extractor.common import InfoExtractor
+
+
+class SamplePluginIE(InfoExtractor):
+    _WORKING = False
+    IE_DESC = False
+    _VALID_URL = r'^sampleplugin:'
+
+    def _real_extract(self, url):
+        self.to_screen('URL "%s" sucessfully captured' % url)