[crunchyroll] Use `enumerate`
Add support for ocw.mit.edu video lectures
Update __init__.py
[nbc] Add an extractor for the main nbc.com site

Some of the videos are encrypted, the f4m downloader doesn’t support them.
Merge pull request #2461 from niebles/master

Update __init__.py

`io` wasn't imported.
[mixcloud] Fix _VALID_RE (fixes #2462)

Accept any character except `/` for uploader and the name, caused problems with non ASCII characters
[mit] Modernize
Merge branch 'master' of github.com:rg3/youtube-dl
Merge branch 'ocw-mit-edu' of https://github.com/amlweems/youtube-dl
[tests] Raise an exception if test definition is invalid (Found in #2460)