[subtitles] refactor to support websites with subtitle information the
webpage.

I added the parameter webpage, so now it's similar to the way automatic
captions are handled. This is an improvement needed for websites like
TED.
[ted] Added support for subtitle download
Merge branch 'ted_subtitles'
Merge remote-tracking branch 'iemejia/master'
[ted] fixed error in case of no subtitles present

I created a test, but I leave it commented since TED videos get
new subtitles frequently.
Merge pull request #1705 from iemejia/master

[ted] support for subtitles
[ted] Return a single info_dict for talks urls

It failed with the --list-subs option
Merge all the subtitles test into a single file

They reuse a base class