Add the ability to process contiguous WebIDL blocks

@darobin

Resolves #334, which also has more discussion of the idea.
[Later processing](https://github.com/jyasskin/respec/blob/8514b97039c160e25325fff3e68aadac0c1b2fd1/js/core/link-to-dfn.js#L23) will remove the unlinked `<a>`. `{{tryLink}}` is more for attaching titles when we've found a definition, and I haven't written anything more than the default `<dfn>` linking to find definitions for the simple interfaces we find in `obj.target` and `obj.implements`.