Added Data attribute function

example/example.go

@@ -12,7 +12,7 @@ func main() {
 			h.Div()(
 				h.Img(h.Src("./me.jpg"), h.Id("profile-photo")),
 				h.Text("hello hatmill!"),
-				h.Div(h.Disabled())(),
+				h.Div(h.Disabled(), h.Data("coolness", "awesome"))(),
 			),
 		),
 	)

hatmill.go

@@ -122,3 +122,11 @@ func WriteDocument(w io.Writer, root *ParentElement) (n int64, err error) {
 	n += nroot
 	return
 }
+
+// Data creates an attribute of the form "data-suffix='value'".
+func Data(suffix, value string) Attrib {
+	return Attrib{
+		Key:   "data-" + suffix,
+		Value: value,
+	}
+}