Support multiple line widths in graphjs

[bwlewis]

The graphjs function only supports a single line width for all edges. This results from the use of a high-performance buffer to define all the edges, see

https://github.com/bwlewis/rthreejs/blob/master/inst/htmlwidgets/scatterplotThree.js#L830

for details. I think that it is possible to support multiple line widths in a high-performance way, but it will probably require GLSL code to do so.

Contributions welcome!