convert multi select demo into reusable component

Currently vivagraph allows to select only one node at time. Sometimes it's useful to be able to select multiple nodes.

https://github.com/anvaka/VivaGraphJS/tree/master/demos/other/webgl-area-select - gives a good starting point of how to do it, but it is not structured as a highly reusable component.

Converting it into component would simplify code reuse

Start by reading the implementation in demos/other/webgl-area-select, which is identified as the starting point for multi-node selection. Extract its behavior into a reusable component so selection can be shared beyond the demo and multiple nodes can be selected.

Written by the indexing model from the issue text.