Follow the spec

[GuiltyDolphin]

It would be great if we could rework the types to better follow the spec. Sure the spec is only a draft, but it should provide a good basis for making sure we aren't missing anything major :)

TODO Keywords

See https://orgmode.org/worg/dev/org-syntax.html#Affiliated_keywords.

Currently, keywords get parsed into an attributes field of elements, but this does not match the spec. Whilst I agree this is a useful transformation, we should be doing this as an optional post step - the AST should match the spec as closely as possible.

• remove attributes field from AST items
• provide an optional post transformation that assigns attributes to their corresponding objects
• ensure only standard keys are supported by default
• provide a parser option to allow additional keys