Meta tag generator

Generates the whole head block at once: title, description, canonical, Open Graph and Twitter card tags, with the card type chosen automatically based on whether there is an image.

Writing these by hand is where inconsistencies creep in — an og:title that drifted from the page title, a twitter:card set to summary while a large image is supplied, a canonical missing entirely. Generating them together keeps them in step. Two notes worth having. The canonical must be an absolute URL and should be self-referential on a normal page; omitting it entirely is usually safer than getting it wrong, since a canonical pointing at the wrong page removes yours from the index. And a robots directive of `index, follow` is the default behaviour, so writing it out achieves nothing — the tag is only worth including when you want `noindex` or `nofollow`.