Enable DAGs / arbitrary graphs

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.

```markdown
# A
- B
- 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.)