Adding persistent option to has_settings

[nicolaiespresso]

Hello,

Thanks for the great work with this gem. It is very useful for us.

In my project, I need to define some settings in a Concern and then another set of settings in the model. Currently, the configuration class resets the default_settings in every has_settings call.
To allow settings additions I added a persistent option. I realize the code could be better. However, initially, I would like you to analyze if this type of solution is acceptable.

Thanks again.

[coveralls]

Coverage increased (+0.09%) to 96.212% when pulling 65f2768 on nicolaiespresso:master into 0edc87f on ledermann:master.

[coveralls]

Coverage increased (+0.07%) to 96.97% when pulling 137ad36 on nicolaiespresso:master into 8888fa1 on ledermann:master.

[ledermann]

Looks good to me, thanks. Just one thing: Please remove the version number change.

[nicolaiespresso]

Ok, just reverted the version file.

[ledermann]

Merged. Thanks for your contribution!