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

youtube_dlc/extractor/__init__.py

@@ -1,13 +1,19 @@
 from __future__ import unicode_literals
 
+from ..utils import load_plugins
+
 try:
     from .lazy_extractors import *
     from .lazy_extractors import _ALL_CLASSES
     _LAZY_LOADER = True
+    _PLUGIN_CLASSES = []
+
 except ImportError:
     _LAZY_LOADER = False
     from .extractors import *
 
+    _PLUGIN_CLASSES = load_plugins('extractor', 'IE', globals())
+
     _ALL_CLASSES = [
         klass
         for name, klass in globals().items()