Add method `frontmatter` to easily extract frontmatter from a parsed document

[haberdashPI]

This is handy if you want to do other things with the front matter once having parsed the Markdown. I didn't see an obvious way to do it from the docs.

[MichaelHatherly]

Thanks @haberdashPI.

This is handy if you want to do other things with the front matter once having parsed the Markdown. I didn't see an obvious way to do it from the docs.

Definitely handy. I've been meaning to look into creating a "proper" API for the AST manipulation/querying. I'm still not 100% sure what it should actually look like though.

My one reservation about adding specialised getters like frontmatter is that I'm unsure where the line would be drawn, do we also want a codeblocks function that returns all the codeblocks in the AST (just as a silly example). Might be worth instead defining a tree manipulation API, maybe just leveraging AbstractTrees.jl for most of it, such that some hypothetical query(FrontMatter, ast) would do the trick, and could extend to more interesting things like map!(uppercase, query(Strong, ast)).

That said, I am fine with just going with the simple frontmatter for now (deprecations are easy enough if something changes in the future). This will need some docs, and tests in that case.

[MichaelHatherly]

CI looks to be failing possibly because there's a frontmatter(::Node) method already defined in src/writers.jl. It does the same thing pretty much as what you've added, so for now you should be able to just use that unexported one.

[haberdashPI]

Hah! Funny that I didn't find that... Well, now it's more discoverable!

And I agree, long-term it does probably make sense to generate some more general-purpose methods for access components of the document programmatically.

[MichaelHatherly]

Even more funny that I completely forgot I had written that function. Thanks for the contribution @haberdashPI.