Allow metadata in the ast passed into the transformer

[RobertDober]

The idea is to adapt Earmark.Transform.transform to accept not only the types emitted by Earmark.to_ast but also tuples of the form {binary|:comment, list, list, %{custom:, _}}

This has two benefits

• It would still allow to extend the ast, e.g. by adding a meta key to the map (e.g. to solve the html tag problem The _HTML Problem_ in the AST #289)
• It allows filters to pass in a filtered / modified ast without an unnecessary and potentially costy cleanup run.