Feature: Parameterize cache store

[oehlschl]

We're looking into the performance of our jbuilder view implementation, and it would be nice to configure the cache store to something besides the Rails store (perhaps ActiveSupport::Cache::NullStore), at least temporarily. It looks like ::Rails.cache is hardcoded in a couple places that could be abstracted to use a class parameter, similar to the way ignore_nil is set up.

I can open a PR if there's other interest or value in this.