node selection cosmetics, interactivity

[nrbgt]

cytoscape doesn't appear to highlight what nodes are actually selected. this can probably be fixed with a selector in their style language. Ideally, we would hoist these to the Base, as e.g. selected_node_color.

Further, both support multiple selection modes: box select is probably the best default, as graphs are big, unless it messes with the pan/zoom interaction. This could also be hoisted as a box_select, or seelction_mode, etc.