Extend SVGAnimatedString with putForwards/stringifier WebIDL behaviors

[AmeliaBR]

As discussed in #175, this would help make SVGAnimatedString behave more like a simple DOMString, e.g. for SVGElement.className.

el.className += " new-class" would work by coercing the SVGAnimatedString to a simple string in order to concatenate it, and then setting the baseVal sub-property when the className property is set to a simple string.

(Not expecting this to happen for SVG 2.0, since we don't have implementations. Although @dstorey said he could make it happen in Edge.)

[AmeliaBR]

See David's comment about implementation, from #175 (comment) :

I'll do another PR to add putForwards/stringifier. I can also implement in Edge. PutForward generates all the code needed automatically so that is a trivial change. stringifier should be straightforward too. At least on baseVal. If we want to add on SVGAElement (probably in href's mixin) to match HTMLAElement.toString() behaviour of returning the href string, then that will need a custom specified stringifier. I believe you do that in WebIDL by specifying new member stringifier and defining the behaviour in prose.