Add MatrixOptionList object

Plus add another accessor method to `CustomFieldList`. I was mapping NetSuite matrix items into Spree product / variants that helps to access the custom fields we care most in those situations.

Sorry for the lack of tests guys =/ NetSuite is giving me such a hard time by the time I should dive in and see how to spec this my motivation is gone. Feel free to let the PR open until I can write some specs when I find that motivation.