Add the ability to process contiguous WebIDL blocks

@darobin

Resolves #334, which also has more discussion of the idea.
@darobin You mentioned the serialization support in https://github.com/darobin/webidl2.js/blob/develop/lib/writer.js. I think this function is the main place I'm generating a text form of a WebIDL construct, and it doesn't seem worth pulling that whole file to get just type serialization.