Add formxxx attributes (for <input type="submit"> element) (#241)

Hello,

A small patch to add the following attributes:

- formaction
- formenctype
- formmethod
- formnovalidate
- formtarget

These attributes are useful in `<input type="submit">` element: [MDN
documentation](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/submit#additional_attributes)

html/attributes.go

@@ -141,10 +141,30 @@ func For(v string) g.Node {
 	return g.Attr("for", v)
 }
 
+func FormAction(v string) g.Node {
+	return g.Attr("formaction", v)
+}
+
 func FormAttr(v string) g.Node {
 	return g.Attr("form", v)
 }
 
+func FormEncType(v string) g.Node {
+	return g.Attr("formenctype", v)
+}
+
+func FormMethod(v string) g.Node {
+	return g.Attr("formmethod", v)
+}
+
+func FormNoValidate() g.Node {
+	return g.Attr("formnovalidate")
+}
+
+func FormTarget(v string) g.Node {
+	return g.Attr("formtarget", v)
+}
+
 func Height(v string) g.Node {
 	return g.Attr("height", v)
 }

html/attributes_test.go

@@ -21,6 +21,7 @@ func TestBooleanAttributes(t *testing.T) {
 		{Name: "controls", Func: Controls},
 		{Name: "defer", Func: Defer},
 		{Name: "disabled", Func: Disabled},
+		{Name: "formnovalidate", Func: FormNoValidate},
 		{Name: "loop", Func: Loop},
 		{Name: "multiple", Func: Multiple},
 		{Name: "muted", Func: Muted},
@@ -61,6 +62,10 @@ func TestSimpleAttributes(t *testing.T) {
 		{Name: "dir", Func: Dir},
 		{Name: "for", Func: For},
 		{Name: "form", Func: FormAttr},
+		{Name: "formaction", Func: FormAction},
+		{Name: "formenctype", Func: FormEncType},
+		{Name: "formmethod", Func: FormMethod},
+		{Name: "formtarget", Func: FormTarget},
 		{Name: "height", Func: Height},
 		{Name: "hidden", Func: Hidden},
 		{Name: "href", Func: Href},