Feature: allow webpack 4 type javascript/esm

In webpack 4 we can set file types such as "javascript/esm" which have more restriction.

With type "javascript/esm" we can't use `require`, `exports` and `module` anymore. But there is still `import`, `export` and `__webpack_module__`

Could it be possible to add a `esm` param to the babel plugin to be able to only use esm import/export
```
// .babelrc
{
"plugins": [["react-hot-loader/babel",{esm:true}]]
}
```