Add build option for es modules

Redux (and a lot of other React libs) have a ES2015 modules version of the build: https://github.com/reactjs/redux/blob/master/package.json#L24

This is really handy for tools like Rollup, since the `"module"` key in package.json can be pointed at `./es/index.js`. I'm using a fork of redux-immutable with these builds and it works well.

Would you consider a PR that adds multiple build outputs to match how Redux is built?

Start with the package.json build configuration and compare the requested outputs with the Redux package.json example linked in the issue. Check how the existing build is produced and how Rollup consumes the package's module entry; done means the package exposes the requested ES2015 module build and matching output entries.

Written by the indexing model from the issue text.