doc: ecma version switcher

[eljefedelrodeodeljefe]

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:

[chrisdickinson]

The resolution on this from the Docs WG was that we weren't going to implement this approach due to the maintenance burden of providing two examples for every existing example in the docs, but instead look into better highlighting for ES2015 syntax.

[chrisdickinson]

(For posterity: the resolution.)

Edit: Sorry for the confusion. I didn't think to comment on the adjoining nodejs.org issue when posting the resolution from the Docs WG meeting, so the resolution didn't get to all of the places it needed to be — mea culpa!

[eljefedelrodeodeljefe]

A pity. Thanks.