Allow to extract a read-only AST with start/end positions attached to every element

[mrkkrp]

This should allow to use MMark parser for syntax highlighting of markdown documents.

[mrkkrp]

Cancelled. The benefits are questionable, because the info that is easily obtainable from the parser (in its current state) is not enough for decent syntax highlighting anyway. On the other hand the added complexity and possible performance regressions may be significant.