Add the ability to process contiguous WebIDL blocks

@darobin

Resolves #334, which also has more discussion of the idea.
I think that when this ships we will want to start warning (just to the console) that `webidl-oldschool` is being deprecated. A few months later escalate that to a visible warning, and later to an error. (And maybe a year from now we can add telemetry to see if we can drop it.)