Add support for third-party extractors

Other packages can register new extractors through the `youtube_dl.extractors`
setuptools entry-point:

    # in mypackage/setup.py
    setup(
        ...
        entry_points={
            'youtube_dl.extractors': [
                'myextractor = mypackage.extractor.MyExtractorIE',
            ],
        },
    )

youtube_dl/extractor/__init__.py

@@ -1,5 +1,6 @@
 from __future__ import unicode_literals
 
+import pkg_resources
 from .abc import ABCIE
 from .academicearth import AcademicEarthCourseIE
 from .addanime import AddAnimeIE
@@ -541,6 +542,7 @@ from .zingmp3 import (
     ZingMp3AlbumIE,
 )
 
+
 _ALL_CLASSES = [
     klass
     for name, klass in globals().items()
@@ -553,7 +555,25 @@ def gen_extractors():
     """ Return a list of an instance of every supported extractor.
     The order does matter; the first extractor matched is the one handling the URL.
     """
-    return [klass() for klass in _ALL_CLASSES]
+
+    # our extractors have priority over external extractors, but
+    # the GenericIE must be inserted later
+    extractors = [klass() for klass in _ALL_CLASSES[:-1]]
+
+    # load external extractors registered throguh the setuptools
+    # entry-point `youtube_dl.extractor.gen_extractors`
+    group = 'youtube_dl.extractors'
+
+    for entrypoint in pkg_resources.iter_entry_points(group=group):
+        # grab the callable that is the actual plugin
+        plugin = entrypoint.load()
+
+        # install the plugin
+        extractors.append(plugin())
+
+    extractors.append(GenericIE())
+
+    return extractors
 
 
 def get_info_extractor(ie_name):