Rewrite to use nicer ES6+, drop old versions of node

[jbt]

This project was originally written in the days of node 0.10 etc so it uses a lot of lowest-common-denominator syntax.

Now that node 4 is out of LTS it would be nice to spruce up the codebase with more modern JS, drop support for anything older than node 6, and cut a 2.0.0 release.

Before doing this, though, it'd be great to know if there'd be any objection to this (although if you're using older unsupported versions of node, the current 1.x.x version will obviously continue to work)