Add the ability to process contiguous WebIDL blocks

@darobin

Resolves #334, which also has more discussion of the idea.
I think we should actually polyfill [`String.prototype.repeat`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/repeat) and then use that, rather than allocating an `Array`, but I was trying not to change webidl-oldschool more than I had to in this PR.