When selecting other languages than 'en', bing-video did not handle the language
correct and gave very bad results. Since User-Agent is normaly rotated in
searx, the behavior of a !biv search was unpredictable and paging was broken.
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>
Add match_language function in utils to match any user given
language code with a list of engine's supported languages.
Also add language_aliases dict on each engine to translate
standard language codes into the custom codes used by the engine.
