Allow host app to predefine product_customization calculators.
Allow host app to predefine product_customization calculators.

jdeen/spree_flexi_variants,jdeen/spree_flexi_variants,renuo/spree_flexi_variants,QuintinAdam/spree_flexi_variants,QuintinAdam/spree_flexi_variants,renuo/spree_flexi_variants,QuintinAdam/spree_flexi_variants,renuo/spree_flexi_variants