ASTNode interface in declaration file lacks a few properties

[zecakeh]

I'm mainly thinking of sourceMeta that allows Markdown-it plugins to add extra data.

Right now we can use attributes to pass extra data to the render rules, but if we want to reuse the plugin with an HTML renderer, the data shows up on the HTML elements.

Is it an oversight or is there a reason to this?

[ahkhanjani]

Added in #132.