Configurable MailChimp API Options

* Made the options passed to the new MC API gem configurable
* Added `Spree::Chimpy.api`. This makes the list + order interface a bit more DRY, and makes it easier to easily grab a handle to the MC API without having to dig out all the right preference values from the spree chimpy config

One thing I did not realize is that with the current MC gem you have to set `throws_exceptions` on the object itself, you can't pass `throws_exceptions: true` in the options hash. [I've created a PR](https://github.com/mailchimp/mailchimp-gem/pull/30) to hopefully fix this issue.