Adding extra config to allow any configuration to be added

[MontyBeda]

Adding param extra_config that will allow any extra configuration to be added to Memcached configuration file. Useful for example to add persistent data when restarting Memcached service but can be used for anything added in the future as well.

[saz]

Thank you for the PR. I'm not really sure, if I like it or not. On the one hand, it will make this module way more flexible, on the other hand, it raises the question "when will a new parameter be added?".
Adding new parameters gives us the possibility of validating things.

Besides that, the REFERENCE.md file is outdated, please run bundle exec rake strings:generate:reference and add the updated REFERENCE.md to your PR.

Currently, this PR is only changing templates/memcached.conf.erb, but other templates must be updated, to add proper support.

[MontyBeda]

It is your module so I let it for your decision. I need it for the data permanence upon server restart only at the moment. But there are few more parameters that cannot be set in the module at the moment and this would just easily enable setting them all up.

If you have the time to add all the parameters then great but this should provide a lot of flexibility to users of the module.

[saz]

@MontyBeda Do you have a list of parameters, which you're currently missing?

[MontyBeda]

I miss only -e for setting ramdisk for cachdisk for Warm reboot. As per https://docs.memcached.org/features/restart/
But we are also thinking about adding TLS for better security as per https://docs.memcached.org/features/tls/ and that would also need few new parameters (-Z -o ssl_chain_cert=/home/user/cert,ssl_key=/home/user/key) and logic around them to use the -o parameter for these options).