doc: ecma version switcher
Picked up from @rvagg's very nice es-swicher on nodejs/nodejs.org#467
This PR adds a switchable box to the docs, with which the user can switch between ecmascript versions. Though this must be extended for an n-th iteration of ecmascript the soulution would be quite clean: The markdown just duplicates content linearly and old browser potentially just render two boxes.
cc/ @nodejs/documentation
TODOs: