[enh] add redis offline engine / https://redis.io/

Slightly modified merge of commit [97269be6], [01a8a581] and [c8d2b5eb] from
searx.

[97269be6] https://github.com/searx/searx/commit/97269be6
[01a8a581] https://github.com/searx/searx/commit/01a8a581
[c8d2b5eb] https://github.com/searx/searx/commit/c8d2b5eb



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>