Enable DAGs / arbitrary graphs

[jabcross]

Would be nice for flowcharts or pipeline diagrams. To be clear, I mean a way of rendering something in this shape:

A___B__C
|\__D_/
 \__E

, maybe with a notation looking something like this, which makes use of HTML anchors.

# A
- B
    - <a id="c"></a> C
- D
    - [c](#c)
- E  

It would still be readable and generate correct HTML in Markdown.

(DAGS would be much easier to implement than arbitrary graphs, by only allowing connections between nodes of different levels.)