Add json endpoint for article

@commuterjoy would overloading the article api (having two different accepts) work with Fastly/Varnish?
Just really FYI, but I've not varied by Accept before as the request headers tend to be quite dirty. Eg, here's my Chrome one ...

`Accept:text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8`

So we risk diluting the cache.

It's fine for now, but I think in the longer term we might either add a file extension or generate a cache key from the incoming Accept header in Varnish and vary by that instead. Stick it on a TODO list?