Expose read-write accessors.

[tmpfs]

For lastLineBlank, htmlBlockType and a data property which allows AST
consumers to associate arbitrary data with an AST node.

[jgm]

I understand about data, but the others are internal state for the parser; why would users need to alter them?

[tmpfs]

I need to know the type of html block type under quite a few scenarios, IIRC particularly to determine if a block is a processing instruction or not. It just feels wrong to be accessing the private variables. I forget for lastLineBlank but i think it was to do with lists at the end of partials and determining whether i need to add and additional line or not. It has been a long time so i would have to read the code again...