New feature : language per search engine !

Adds engines : Youtube with or without API and multiple Qwant

Add startpage as an autocompleter engine

google engine

implement swisscows engine

TODO: implement image engine

- Web
- Images
- News
- Social media

The API needs an API_KEY
The NOAPI doesn't have the published dates.

This allow to send request to google.* (according to the search language).
Before this commit, request in other languages than english was sent to www.google.com which was redirected to www.google.*
The PREF is still use on the www.google.com domain.

[fix] flickr engine

[fix] bing_news - fixes #328

[enh] currency_convert engine : "1 dollars in euros"

The currency names are fetched wikidata and store into a static file : searx/data/currencies.json
This file is loaded when the currency_converter is loaded.
A database is perhaps more appropiated.

[fix] yahoo_news engine

update versions.cfg to use the current up-to-date packages

[fix] twitter engine

[fix] Flickr engine

[enh] reduce the number of http outgoing connections.

Revert of #195 when the search language is not english