Add interpolation? check for RegexpNode

Closes rubocop#4

lib/rubocop/ast/node/regexp_node.rb

@@ -31,6 +31,11 @@ def regopt
       def content
         children.select(&:str_type?).map(&:str_content).join
       end
+
+      # @return [Bool] if regexp contains interpolation
+      def interpolation?
+        children.select(&:begin_type?).any?
+      end
     end
   end
 end

spec/rubocop/ast/regexp_node_spec.rb

@@ -140,4 +140,24 @@
       it { expect(content).to eq("\n.+\n") }
     end
   end
+
+  describe '#has_interpolation?' do
+    context 'with direct variable interpoation' do
+      let(:source) { '/\n\n#{foo}(abc)+/' }
+
+      it { expect(regexp_node.interpolation?).to eq(true) }
+    end
+
+    context 'with regexp quote' do
+      let(:source) { '/\n\n#{Regexp.quote(foo)}(abc)+/' }
+
+      it { expect(regexp_node.interpolation?).to eq(true) }
+    end
+
+    context 'with no interpolation returns false' do
+      let(:source) { '/\n\n(abc)+/' }
+
+      it { expect(regexp_node.interpolation?).to eq(false) }
+    end
+  end
 end