support render in node.js

[seven1240]

First thanks to the great mind and great software.

similar to #66 , I'd like to render to svg/png in node.js, but I tried jsdom and svg.js, all failed, maybe because they are not real DOMs.

seems markmap use d3.js, and d3.js seems support jsdom. anyway, render mindmap to svg doesn't need a real dom. is it possible to make it work in node.js ?

Thanks.