Add an extractor for common mistakes (#4610)

youtube_dl/extractor/__init__.py

@@ -71,6 +71,7 @@ from .cnn import (
 from .collegehumor import CollegeHumorIE
 from .comedycentral import ComedyCentralIE, ComedyCentralShowsIE
 from .comcarcoff import ComCarCoffIE
+from .commonmistakes import CommonMistakesIE
 from .condenast import CondeNastIE
 from .cracked import CrackedIE
 from .criterion import CriterionIE