Add an extractor for space.com (fixes #1718)

It uses Brightcove, but requires some special process for getting a url with the playerKey field in some videos

youtube_dl/extractor/__init__.py

@@ -115,6 +115,7 @@ from .slideshare import SlideshareIE
 from .sohu import SohuIE
 from .soundcloud import SoundcloudIE, SoundcloudSetIE, SoundcloudUserIE
 from .southparkstudios import SouthParkStudiosIE
+from .space import SpaceIE
 from .spankwire import SpankwireIE
 from .spiegel import SpiegelIE
 from .stanfordoc import StanfordOpenClassroomIE