Add support for ES6 module import

[LostInBrittany]

Hi!

I think it would be nice if select was available as an ES6 module. The hidden goal for me is having clipboard.js available as an ES6 module (fixing zenorocha/clipboard.js#568).

In a first time, you could simply expose an module/select.js file and declare it in a module field in package.json. And then, in a second time, you could use the ES6 version as the baseline and generate the commonJS module via Rollup...

What do you think?