Add the ability to process contiguous WebIDL blocks

@darobin

Resolves #334, which also has more discussion of the idea.
No, because [generating the children](https://github.com/jyasskin/respec/blob/8514b97039c160e25325fff3e68aadac0c1b2fd1/js/core/webidl-contiguous.js#L355) uses `.join(",\n")`, which leaves off the last newline.