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

Slightly modified merge of commit [97269be6], [01a8a5814a] 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>

searx/settings.yml

@@ -960,6 +960,15 @@ engines:
     timeout : 10.0
     disabled : True
 
+  # Required dependency: redis
+  #  - name: myredis
+  #    engine: redis_server
+  #    exact_match_only: False
+  #    host: '127.0.0.1'
+  #    port: 6379
+  #    password: ''
+  #    db: 0
+
 # tmp suspended: bad certificate
 #  - name : scanr structures
 #    shortcut: scs