Added base/normal documents and standard features

ok, so this is yet another alternative to the already discused **FullFeatures** approach.

This one is better suited to things like the `BlockBundle` where we don't necesisarily want to have 20 documents instead of 10. By default documents would be *feature complete*, but important documents should extend a `Base` document as described in this proposal.

This has the downside that we cannot type-check for an "Advanced" document - but this shouldn't matter in the long term as we should be checking for the existance of individual features.