Rectify HBMustacheContext methods odd and even

Sources/HummingbirdMustache/Context.swift

@@ -30,6 +30,10 @@ struct HBMustacheContext: HBMustacheMethods {
             return self.last
         case "index":
             return self.index
+        case "even":
+            return (self.index & 1) == 0
+        case "odd":
+            return (self.index & 1) == 1
         default:
             return nil
         }