Bring back the formatter that was unexpectedly removed

closes #103

lib/did_you_mean.rb

@@ -50,4 +50,20 @@ def self.formatter=(formatter)
   end
 
   self.formatter = PlainFormatter.new
+
+  # Deprecated formatter
+  class Formatter #:nodoc:
+    def initialize(corrections = [])
+      @corrections = corrections
+    end
+
+    def to_s
+      return "" if @corrections.empty?
+
+      output = "\nDid you mean?  ".dup
+      output << @corrections.join("\n               ")
+    end
+  end
+
+  deprecate_constant :Formatter
 end

test/deprecated_formatter_test.rb

@@ -0,0 +1,9 @@
+require 'test_helper'
+
+class DeprecatedFormatterTest < Minitest::Test
+  def test_deprecated_formatter
+    assert_output nil, "test/deprecated_formatter_test.rb:6: warning: constant DidYouMean::Formatter is deprecated\n" do
+      assert_equal "\nDid you mean?  does_exist", ::DidYouMean::Formatter.new(['does_exist']).to_s
+    end
+  end
+end