Add support for arbitrary block attributes.

[hayleigh-dot-dev]

Jot already supports class and id attributes on blocks using {.wibble} and {#wobble} respectively. Djot additionally supports arbitrary attributes attached to blocks like so:

{source="Iliad"}
> Sing, muse, of the wrath of Achilles

Lustre's docs would make heavy use of these in its API reference to mark which platforms the thing being documented is available on:

☝️ This would be written as

{api=true}
{target=erlang}
{target=javascript}
### Error

---

Context: I think I might be the only one properly using jot right now, so I'm opening issues here in case anyone wants to implement bits of the spec that will have immediate impact.