[espn] Add extractor (#4396)

Unfinished

youtube_dl/extractor/__init__.py

@@ -141,6 +141,7 @@ from .engadget import EngadgetIE
 from .eporner import EpornerIE
 from .eroprofile import EroProfileIE
 from .escapist import EscapistIE
+from .espn import ESPNIE
 from .everyonesmixtape import EveryonesMixtapeIE
 from .exfm import ExfmIE
 from .expotv import ExpoTVIE