Allow custom association name for subscriptions

[apuntovanini]

Issue #161, if available:

Summary

Right now I just added a failing spec for custom association name. I assumed we can use the same config for custom subscriptions table, but of course I'll let you decide this @simukappu

There are many methods inside the subscriber.rb that depends on the default subscriptions association name. As said, if anyone attaches this concern to a model that has subscriptions associations, it breaks functionality

Please give me instructions on the direction you prefer to move forward about this
Thanks

[apuntovanini]

I took a stronger path on this, since we need this functionality for production shortly. I created a fork in which I renamed associations and methods from :subscription(s) to :notification_subscription. I know it's a bit opinionated, but naming it :subscription is just about the same. If you think this will work for the gem, let me know, I'll wrap the PR against master branch
Thanks!