Add fragment element

[goring]

Added a new fragment element that is purely for ergonomics, allowing the ability to group elements without adding an extra wrapper element to the DOM.

I found this to be useful when I have functions that return nodes that need to be merged into the same parent without any element wrapping it.

Example Use Case

func TestFragment(t *testing.T) {
	expected := `<div><p>0</p><p>1</p><p>2</p><p>3</p><p>4</p></div>`
	nodes1 := []Node{
		P(nil,
			Text("1"),
		),
		P(nil,
			Text("2"),
		),
	}
	nodes2 := []Node{
		P(nil,
			Text("3"),
		),
		P(nil,
			Text("4"),
		),
	}
	el := Div(nil,
		P(nil,
			Text("0"),
		),
		Fragment(nodes1...),
		Fragment(nodes2...),
	)
	actual := el.Render()
	assert.Equal(t, expected, actual)
}

[chasefleming]

Great idea! Thanks for adding this.