For some extractors that are hard to workout a good _VALID_URL we use very vague and unrestrictive ones,
e.g. just allowing anything after hostname and capturing part of URL as id.
If some of these extractors happen to have an video embed of some different hoster or platform
and this scenario was not handled in extractor itself we end up with inability to download this embed
until extractor is fixed to support embed of this kind.
Forcing downloader to use the generic extractor can be a neat temporary solution for this problem.
Example: FiveTV extractor with Tvigle embed - http://www.5-tv.ru/rabota/broadcasts/48/
It doesn't work well with 'bestvideo' and 'bestaudio' because they are usually before the max quality.
Format filters should be used instead, they are more flexible and don't require the requested quality to exist for each video.
* Remove the 'songtitle' field, 'title' can be used instead.
* Remove newlines in the help text, for consistency with other options.
* Add 'from __future__ import unicode_literals'.
* Call '__init__' from the parent class.
* Add test for the format_to_regex method
For each language the extractor builds a list with the available formats sorted (like for video formats), then YoutubeDL selects one of them using the '--sub-format' option which now allows giving the format preferences (for example 'ass/srt/best').
For each format the 'url' field can be set so that we only download the contents if needed, or if the contents needs to be processed (like in crunchyroll) the 'data' field can be used.
The reasons for this change are:
* We weren't checking that the format given with '--sub-format' was available, checking it in each extractor would be repetitive.
* It allows to easily support giving a format preference.
* The subtitles were automatically downloaded in the extractor, but I think that if you use for example the '--dump-json' option you want to finish as fast as possible.
Currently only the ted extractor has been updated, but the old system still works.
