Add ability to set CSS classes instead of inline styles (#103)

* Add ability to set CSS classes instead of inline styles

This allows to set a `ClassName` field in `Style` structs. Setting this
field to anything but "" will cause all other styles to be ignored. The
element will then have a `class=` tag instead with the corresponding name.

Possible reasons to use this:
* Including multiple SVGs on the same webside, using the same styles
* Desire to use strict CSP headers

* Add warning that setting `ClassName` will drop all other inline styles

raster_renderer.go

@@ -49,6 +49,10 @@ func (rr *rasterRenderer) SetDPI(dpi float64) {
 	rr.gc.SetDPI(dpi)
 }
 
+// SetClassName implements the interface method. However, PNGs have no classes.
+func (vr *rasterRenderer) SetClassName(_ string) {
+}
+
 // SetStrokeColor implements the interface method.
 func (rr *rasterRenderer) SetStrokeColor(c drawing.Color) {
 	rr.s.StrokeColor = c