Allow omitting the Tags block, if no tags needed
Documents' metadata fields can be in any order

It'd be a big ol' pain to have to remember the fairly-arbitrary order I
initially defined for the metadata fields. This should be much more
user-friendly. And when the user is me, I want to be friendly to him :~O
Infer the slug from the title, if not provided

This is another user-friendliness thing. There's no sense requiring
users to duplicate their title in the usual case.
Reorder dependencies in test.build-depends

This makes it easier to tell at a glance what's where. The sorting is
implementation-necessary then test-specific, and alphabetical inside
that.