Block frontmatter

Instead of the --- lines around the frontmatter, a slide can start with a fenced yaml block. It is useful when your editor treats --- as something else.

---

```yaml
layout: center
class: text-center
```

# A centred slide

The block is read by the markdown parser, which is Slidev's own, and gives the same result as the usual form. It must be at the very top of the slide.