I found that currently only tagesschau videos are played. There are some
more shows hosted on tagesschau.de (see [0] for example) which are
easily playable by adjusting the regex. So this patch adds support for:
* tagesthemen
* tagesschau vor 20 Jahren
* tagesschau (mit Gebärdensprache)
* nachtmagazin
Note that some videos don't provide a description, so in order for the
tests to succeed, an ExtractorError needs to get caught.
[0]: http://www.tagesschau.de/multimedia/video/videoarchiv2~_date-20150714.html
