[animalplanet] Add new extractor(closes #5303)

youtube_dl/extractor/__init__.py

@@ -19,6 +19,7 @@ from .aftonbladet import AftonbladetIE
 from .airmozilla import AirMozillaIE
 from .aljazeera import AlJazeeraIE
 from .alphaporno import AlphaPornoIE
+from .animalplanet import AnimalPlanetIE
 from .anitube import AnitubeIE
 from .anysex import AnySexIE
 from .aol import AolIE