Requesting support for tspan
Thanks for the useful library, one thing I found missing (and had to hack together myself) was support for tspan - as I was trying to center text with mixed styling.

This is what I ended up with in non-method form (the _svg functions are my replicas of the svgo private methods)

``` golang
func startText(canvas *svg.SVG, x int, y int, s ...string) {
	_svg_printf(canvas, `<text %s %s`, _svg_loc(x, y), _svg_endstyle(s, ">"))
}

func addText(canvas *svg.SVG, t string, s ...string) {
	if len(s) == 0 {
		xml.Escape(canvas.Writer, []byte(t))
		return
	}
	_svg_printf(canvas, `<tspan %s`, _svg_endstyle(s, ">"))
	xml.Escape(canvas.Writer, []byte(t))
	_svg_printf(canvas, "</tspan>")
}

func endText(canvas *svg.SVG) {
	_svg_println(canvas, "</text>")
}
```