Node.js render example

[mattsrinc]

This is just a suggestion to include more thorough Node.js usage example.

And an expanded example if several exports from the same image are required would also be helpful. Maybe here could be a possible improvement (not rereading the same input file again and again).

Lastly timing comparison between calling svgexport command as a process and calling it by providing JSON parameters (both in Node.js app) should be there for a big picture. After all when you have 1365 images to create from one SVG image (typical Leaflet map with 5 zoom levels) there's a huge difference between 8 and 14 hours of rendering and exporting time.