tree: explore alternative graph implementations

[dominicbarnes]

From @dominicbarnes on February 11, 2017 3:58

The current graph.js implementation has worked well, but overall it's grown rather stale and hasn't addressed some of it's weird API quirks. In addition, it's ES6-first distribution has caused some headaches around distributing this library. (looking at you, babel-polyfill)

I recently came across graphology in some articles I was reading, and would like to try incorporating it here to see if it can be a suitable replacement.

Copied from original issue: makojs/tree#40