add odd? and even? to the loop tag (#232)

* add `odd?` and `even?` to the loop tag

* use crystal native odd/even. update spec

spec/marten/template/tag/for/loop_spec.cr

@@ -101,4 +101,44 @@ describe Marten::Template::Tag::For::Loop do
       loop.revindex0.should eq 8
     end
   end
+
+  describe "#odd?" do
+    it "returns true if the index is odd" do
+      loop = Marten::Template::Tag::For::Loop.new(items_size: 10)
+      loop.index = 1
+      loop.odd?.should eq true
+
+      loop.index = 3
+      loop.odd?.should eq true
+    end
+
+    it "return false if the index is even" do
+      loop = Marten::Template::Tag::For::Loop.new(items_size: 10)
+      loop.index = 2
+      loop.odd?.should eq false
+
+      loop.index = 4
+      loop.odd?.should eq false
+    end
+  end
+
+  describe "#even?" do
+    it "returns true if the index is even" do
+      loop = Marten::Template::Tag::For::Loop.new(items_size: 10)
+      loop.index = 0
+      loop.even?.should eq true
+
+      loop.index = 2
+      loop.even?.should eq true
+    end
+
+    it "return false if the index is odd" do
+      loop = Marten::Template::Tag::For::Loop.new(items_size: 10)
+      loop.index = 1
+      loop.even?.should eq false
+
+      loop.index = 3
+      loop.even?.should eq false
+    end
+  end
 end

src/marten/template/tag/for/loop.cr

@@ -36,6 +36,14 @@ module Marten
           def revindex0
             @items_size - @index - 1
           end
+
+          def odd?
+            @index.odd?
+          end
+
+          def even?
+            @index.even?
+          end
         end
       end
     end