Let user force nodes to stay in specific positions

[nowthis]

Follow-up to issue #5.

Currently: after any nodes are dragged into new positions, any re-rendering of the graph will re-set the nodes to their original positions.

Add the following capabilities:

• Add a switch so that when previewing changes to the graph, the nodes will not move from their present positions -- only the node & flow sizes will change.
  • If new flows or nodes are added to the graph's source data, this may be difficult to support -- they have to go somewhere. Possibly the capability should only work when flows are being modified, not added.
  • Deleted flows don't present the same problem, they can just disappear.
• When saving a graph's specification, include the current positions of the nodes.
• When rendering a graph from a saved spec, force the nodes into the specified positions. (Always? I think so, yes.)

[nowthis]

Closing/won't do. I don't think this is super useful, in hindsight. Other controls over layout will help more people than this sort of complex option, plus I'm very dubious that you could reasonably keep anything in place well when node values change.