add ability to ignore keys, closes #22

Immigrant.ignore_keys allows you to specify a list of keys that should be
ignored (both in the migration generator and the rake task). this is
useful if you have associations spanning schemas or databases (see #21)

just create an config/initializers/immigrant.rb file with something like
the following:

    Immigrant.ignore_keys = [
      { from_table: "users", column: "account_id" },
      # etc
    ]

note that you need to specify both the from_table and the column

lib/immigrant.rb

@@ -1,13 +1,28 @@
 require 'active_support/all'
 
 module Immigrant
+  class << self
+    # expected format:
+    # [{from_table: "the_table", column: "the_column"}, ...]"
+    attr_writer :ignore_keys
+
+    def ignore_keys
+      @ignore_keys ||= []
+    end
+  end
+
   class KeyFinder
     def infer_keys(db_keys = current_foreign_keys, classes = model_classes)
-      database_keys = db_keys.inject({}) { |hash, foreign_key|
-        hash[foreign_key.hash_key] = foreign_key
-        hash
-      }
+      database_keys = Hash[db_keys.map { |foreign_key|
+        [foreign_key.hash_key, foreign_key]
+      }]
+
+      ignore_keys = Hash[Immigrant.ignore_keys.map { |key|
+        [[key[:from_table], key[:column]], true]
+      }]
+
       model_keys, warnings = model_keys(classes)
+
       new_keys = []
       model_keys.keys.each do |hash_key|
         foreign_key = model_keys[hash_key]
@@ -18,7 +33,7 @@ def infer_keys(db_keys = current_foreign_keys, classes = model_classes)
           if current_key.to_table != foreign_key.to_table || current_key.options[:primary_key] != foreign_key.options[:primary_key]
             warnings[hash_key] = "Skipping #{foreign_key.from_table}.#{foreign_key.options[:column]}: its association references a different key/table than its current foreign key"
           end
-        else
+        elsif !ignore_keys[hash_key]
           new_keys << foreign_key
         end
       end

test/immigrant_test.rb

@@ -455,6 +455,30 @@ class Widget < ActiveRecord::Base
     end
   end
 
+  test 'ignore_keys should be respected' do
+    given <<-CODE
+      class User <  ActiveRecord::Base; end
+      class Category < ActiveRecord::Base; end
+      class Widget < ActiveRecord::Base
+        belongs_to :user
+        belongs_to :category
+      end
+    CODE
+
+    Immigrant.ignore_keys = [{:from_table => "widgets", :column => "category_id"}]
+    begin
+      assert_equal(
+        [foreign_key_definition(
+           'widgets', 'users',
+           :column => 'user_id', :primary_key => 'id'
+         )],
+        infer_keys
+      )
+    ensure
+      Immigrant.ignore_keys = []
+    end
+  end
+
   test 'ForeignKeyDefinition#to_ruby should correctly dump the key' do
     if ActiveRecord::VERSION::STRING < '4.2.'
       definition = foreign_key_definition('foos', 'bars', dependent: 'delete')