Add experimental support for lazy loading the info extractors

'make lazy-extractors' creates the youtube_dl/extractor/lazy_extractors.py (imported by youtube_dl/extractor/__init__.py), which contains simplified classes that only have the 'suitable' class method and that load the appropiate class with the '__new__' method when a instance is created.

devscripts/lazy_load_template.py

@@ -0,0 +1,17 @@
+# flake8: noqa
+from __future__ import unicode_literals
+
+import re
+
+
+class LazyLoadExtractor(object):
+    _module = None
+
+    @classmethod
+    def ie_key(cls):
+        return cls.__name__[:-2]
+
+    def __new__(cls):
+        mod = __import__(cls._module, fromlist=(cls.__name__,))
+        real_cls = getattr(mod, cls.__name__)
+        return real_cls.__new__(real_cls)