support es6 modules

[kmptkp]

Now that node supports native javascript modules (behind an experimental flag, on its way to becoming default), would there be interest in having doctest also allow native javascript module instead of requiring commonjs/amd?

[davidchambers]

I'd like to see this. ES5 is my favourite flavour of JavaScript, so I don't have much use for this myself, but if someone would like to submit a pull request I'll happily review it. :)