Add support for ES6 module import

[LostInBrittany]

Currently I'm developing several webcomponents using Polymer 3 and LitElement. Both of them rely on ES6 imports and leverage on the browser ES6 modules import capabilities.

For one of my components I have a dependency on a lib that depends on tiny-emitter. I would need tiny-emitter to be available also as an ES6 module.

I'm making a PR to propose it...

[scottcorgan]

Sounds great to me, looking forward to it. Thanks!

[hit12369]

I've made a very small (< 0.3 k) ES module compatible event emitter using native EventTarget.
https://github.com/hkk12369/pico-emitter