[enh] add redis connector searx/shared/redisdb.py

Add a redis connector, the default DB connector is a socket at::

    unix:///usr/local/searxng-redis/run/redis.sock?db=0

To set up a redis instance simply use::

    $ ./manage redis.build
    $ sudo -H ./manage redis.install

A hint for developers:

To get access rights to this instance, your developer account needs to be added
to the *searxng-redis* group::

    $ sudo -H ./manage redis.addgrp "${USER}"
    # don't forget to logout & login to get member of group

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

docs/src/searx.shared.redisdb.rst

@@ -0,0 +1,8 @@
+.. _redis db:
+
+========
+Redis DB
+========
+
+.. automodule:: searx.shared.redisdb
+  :members: