Add Threaded I/O options to config

[calvinbui]

Redis 6 has some new options for multithreading. They can be seen at http://download.redis.io/redis-stable/redis.conf in the Threaded I/O section.

I can work on this but I was thinking to add the ability to add more configuration options. Something like:

vars:

redis_config_additional: |
  io-threads 4
  io-threads-do-reads yes

with the default option being ""

jinja template:

...
{% if redis_config_additional|length %}
# Additional
{{ redis_config_additional }}
{% endif %}
...

This will allow users to add their own config without the role having to manually specify new ones all the time

[DavidWittman]

+1 this would resolve #105 too