use canvas instead of svg as a default or an option

enable, by default or as an option, plotting using canvas versus svg

I have a working prototype of `forceNetwork()` working that uses d3 v4 and plots using canvas instead of svg, so it's at least feasible. Apparently, it would afford significant performance gains when plotting with many nodes. Not certain if or what issues this might cause. Thoughts and ideas are appreciated. Maybe I'll get a branch on my fork working and link to it here so people can experiment with it?

(preemptive issue, anticipating/depending on the completed conversion to d3 v4 #143)