Move redis.c from credis to hiredis
Add support for custom commands in redis.

Now the redis (hiredis based) plugin support custom queries to get data,
for example:

<Query "name">
	Command "LLEN key"
	Type "gauge"
</Query>
Change redis collect behaviour.

According to octo comments in:
 https://github.com/collectd/collectd/pull/296#discussion_r3771870

- Remove info struct and parse info in pipeline
- Handle DS_TYPE_ABSOLUTE and DS_TYPE_DERIVE
- Removed dashed in plugin name
- Some other minor changes
Add query example in collectd.conf
Add documentation to collectd.conf