[loc] Add extractor (Closes #3188)

Added extractor of loc.gov, which closes #3188. I am not an experienced programmer, so I am sure I did a bunch of mistakes, but the extractor works (for me at least).

[LibraryOfCongress] don't use video_id for _search_regex()

[LibraryOfCongress] Improvements

youtube_dl/extractor/extractors.py

@@ -382,6 +382,7 @@ from .leeco import (
     LePlaylistIE,
     LetvCloudIE,
 )
+from .libraryofcongress import LibraryOfCongressIE
 from .libsyn import LibsynIE
 from .lifenews import (
     LifeNewsIE,