Document parser hooks

[mixonic]

Introduce a public API for parser hooks.

TODO

• Get general 👍 from @bantic
• Tests
• Refactor to the documented API
• Port existing hook usage here and in TypeSet, smoke test

Fixes #229

[bantic]

appropriate

[bantic]

few typos, but otherwise these docs look 👍 to me. There are some scenarios where a return value may be invalid, however. For instance, if a parser hook returned a marker where a section was expected, I expect the parsing would break later (perhaps much later, at render time) when attempting to deal with a post that had a list of sections that looked like [markupSection, listSection, marker].

Is there a plan to guard against nonsensical (in context) return values for parser hooks?

[mixonic]

@bantic perhaps we could assert the correct type is appended to a linked-list during adoptItem