Support custom GTINs

This refactors GTIN handler class lookup so it can be extended without
monkeypatching. Developers can add/remove classes in
`BarcodeValidation::GTIN.gtin_classes` to control which class wraps a
GTIN. The use case I need this for is internall dummy GTINs that are
longer than any of the standard ones.

The Readme is updated to describe the feature.

README.md

@@ -63,6 +63,32 @@ bad.to_all_valid         # => []
 ```
 
 
+Custom GTINs
+------------
+
+If the standard GTINs provided are not enough for your needs, you can implement your own and register it.
+`BarcodeValidation::GTIN.gtin_classes` contains the ordered list of GTIN classes that are checked if they `handle?(input)`.
+Add your own class to the list, or remove default ones, to tailor it to your needs. For example:
+
+```ruby
+# A custom class that handles any length GTIN as long as it starts with "123".
+# Note that we must still provide a VALID_LENGTH to allow transcoding to other GTINs by zero-padding.
+class MyCustomGTIN < BarcodeValidation::GTIN::Base
+  VALID_LENGTH = 20
+
+  def self.handles?(input)
+    input.start_with?("123") && input.length <= VALID_LENGTH
+  end
+
+  # Custom validity check
+  def valid?
+    self.class.handles?(input) && check_digit.valid?
+  end
+end
+
+BarcodeValidation::GTIN.gtin_classes.unshift MyCustomGTIN
+```
+
 
 Development
 -----------

lib/barcodevalidation/gtin.rb

@@ -12,17 +12,19 @@
 module BarcodeValidation
   module GTIN
     class << self
+      attr_accessor :gtin_classes
+
       def new(input)
         (class_for_input(input) || BarcodeValidation::InvalidGTIN).new(input)
       end
 
       private
 
       def class_for_input(input)
-        [GTIN8, GTIN12, GTIN13, GTIN14].find do |klass|
-          input.to_s.size == klass::VALID_LENGTH
-        end
+        input = input.to_s.freeze
+        gtin_classes.find { |klass| klass.handles?(input) }
       end
     end
+    self.gtin_classes = [GTIN8, GTIN12, GTIN13, GTIN14]
   end
 end

lib/barcodevalidation/gtin/base.rb

@@ -19,6 +19,14 @@ def initialize(input)
         BarcodeValidation::InvalidGTIN.new(input, error: e)
       end
 
+      # Does this class (potentially) handle a GTIN that matches the input?
+      # Subclasses can choose to implement their own logic. The default is to look at +VALID_LENGTH+ and use that to match the length of the input the class handles.
+      def self.handles?(input)
+        return false unless self.const_defined?(:VALID_LENGTH)
+
+        input.length == self::VALID_LENGTH
+      end
+
       def valid?
         valid_length == length && check_digit.valid?
       end