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

renderer.go

@@ -18,6 +18,9 @@ type Renderer interface {
 	// SetDPI sets the DPI for the renderer.
 	SetDPI(dpi float64)
 
+	// SetClassName sets the current class name.
+	SetClassName(string)
+
 	// SetStrokeColor sets the current stroke color.
 	SetStrokeColor(drawing.Color)