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.

youtube_dl/extractor/__init__.py

@@ -1,13 +1,17 @@
 from __future__ import unicode_literals
 
-from .extractors import *
+try:
+    from .lazy_extractors import *
+    from .lazy_extractors import _ALL_CLASSES
+except ImportError:
+    from .extractors import *
 
-_ALL_CLASSES = [
-    klass
-    for name, klass in globals().items()
-    if name.endswith('IE') and name != 'GenericIE'
-]
-_ALL_CLASSES.append(GenericIE)
+    _ALL_CLASSES = [
+        klass
+        for name, klass in globals().items()
+        if name.endswith('IE') and name != 'GenericIE'
+    ]
+    _ALL_CLASSES.append(GenericIE)
 
 
 def gen_extractor_classes():