gh-1514

This fixes #1514 - previously, `<!DOCTYPE>` declarations were handled as just another type of element, which was a bit of a hack. Turns out this doesn't work with a lot of doctype declarations, so rather than piling hacks upon hacks, this PR introduces a new doctype primitive.

It means bumping the template version, which to be honest should probably already have happened by now. AFAIK this shouldn't affect anything else (e.g. component loaders), it just means that users of Ractive 0.7.x will need to ensure that their server/build process (if any) is also up-to-date (which is exactly the point of versioning).

I'll update https://github.com/ractivejs/template-spec before merging this, otherwise I'll probably forget and never get round to it...