Add configuration for discount calculator

This allows stores to specify their own discount calculator in an
initializer, by doing something like the following:

SuperGood::SolidusTaxJar.discount_calculator = MyStore::CustomDiscountCalculator

The class's constructor should take in a line item and have a #discount
method that returns the discount amount (which should be a negative
number.)

1 files changed, 10 insertions, 4 deletions

diff --git a/lib/super_good/solidus_taxjar.rb b/lib/super_good/solidus_taxjar.rb
index 40df3ec..f525ee7 100644
--- a/lib/super_good/solidus_taxjar.rb
+++ b/lib/super_good/solidus_taxjar.rb
@@ -2,11 +2,17 @@ require 'solidus_core'
 require 'solidus_support'
 require 'taxjar'
 
+require "super_good/solidus_taxjar/version"
+require "super_good/solidus_taxjar/api"
+require "super_good/solidus_taxjar/tax_calculator"
+require "super_good/solidus_taxjar/discount_calculator"
+
 module SuperGood
   module SolidusTaxJar
+    class << self
+      attr_accessor :discount_calculator
+    end
+
+    self.discount_calculator = ::SuperGood::SolidusTaxJar::DiscountCalculator
   end
 end
-
-require "super_good/solidus_taxjar/version"
-require "super_good/solidus_taxjar/api"
-require "super_good/solidus_taxjar/tax_calculator"