layout algorithm directedness?

[gordonwoodhull]

In graphviz, the format of the file (whether the first line is digraph or graph) determines whether -> or -- arrows are allowed, and whether dir will default to none or normal.

This is neat but

1. we normally deal with json, which makes no such distinction
2. currently we don't have any graph-level attributes in json (although maybe we will someday)
3. graphlib-dot supplies isDirected but we throw it out since we pretend graphviz is a special case of json (!)

I dunno, maybe this is misguided, but the proposal is that maybe directedness is a property of the layout algorithm, and we could default dir by reading a flag on the algorithm?

Anyway, right now simple-viewer gives inconsistent results depending whether you start in graphviz mode or not, and something should be done!