Add web component elements and slot attribute (#235)

The slot attribute is a standard attribute for use with web components:
https://developer.mozilla.org/en-US/docs/Web/API/Web_components/Using_templates_and_slots

html/attributes.go

@@ -133,6 +133,10 @@ func DataAttr(name, v string) g.Node {
 	return Data(name, v)
 }
 
+func SlotAttr(v string) g.Node {
+  return g.Attr("slot", v)
+}
+
 func For(v string) g.Node {
 	return g.Attr("for", v)
 }

html/attributes_test.go

@@ -84,6 +84,7 @@ func TestSimpleAttributes(t *testing.T) {
 		{Name: "role", Func: Role},
 		{Name: "rows", Func: Rows},
 		{Name: "rowspan", Func: RowSpan},
+		{Name: "slot", Func: SlotAttr},
 		{Name: "src", Func: Src},
 		{Name: "srcset", Func: SrcSet},
 		{Name: "step", Func: Step},

html/elements.go

@@ -264,6 +264,10 @@ func Select(children ...g.Node) g.Node {
 	return g.El("select", children...)
 }
 
+func SlotEl(children ...g.Node) g.Node {
+	return g.El("slot", children...)
+}
+
 func Source(children ...g.Node) g.Node {
 	return g.El("source", children...)
 }
@@ -296,6 +300,10 @@ func Td(children ...g.Node) g.Node {
 	return g.El("td", children...)
 }
 
+func Template(children ...g.Node) g.Node {
+	return g.El("template", children...)
+}
+
 func Textarea(children ...g.Node) g.Node {
 	return g.El("textarea", children...)
 }

html/elements_test.go

@@ -103,6 +103,7 @@ func TestSimpleElements(t *testing.T) {
 		{Name: "script", Func: Script},
 		{Name: "section", Func: Section},
 		{Name: "select", Func: Select},
+		{Name: "slot", Func: SlotEl},
 		{Name: "small", Func: Small},
 		{Name: "span", Func: Span},
 		{Name: "strong", Func: Strong},
@@ -114,6 +115,7 @@ func TestSimpleElements(t *testing.T) {
 		{Name: "table", Func: Table},
 		{Name: "tbody", Func: TBody},
 		{Name: "td", Func: Td},
+		{Name: "template", Func: Template},
 		{Name: "textarea", Func: Textarea},
 		{Name: "tfoot", Func: TFoot},
 		{Name: "th", Func: Th},