Add preference for whether store credit should be automatically applied

[mleglise]

My current project wants the customer to be able to choose whether or not to apply their store credit to a given order. I'll be setting this up for my own project, but would there be interest if I made it into a PR for this gem? Not sure how that would overlap with the effort to move some pieces into Spree::Core.

[athal7]

PR would be great, maybe for core though after that PR gets merged in would be better?

[JDutil]

Yea I would wait and PR against core.