Add a configuration option to map class name to module exports in ES module mode

[joseprio]

First of all, thanks for fixing #424!

Feature Proposal

At the moment, a dictionary object is exported in the generated JS module that maps the old classname with the generated one. Unfortunately, this prevents mangling those keys, as it's not guaranteed that they change during runtime (although it's reasonable to assume they won't).
We can solve this by exporting each of those classnames as a module export. Obviously, there's a problem with classnames with hyphens, so I definitely see this as a disabled-by-default configuration flag that would just throw with an informative message if it encounters classnames with dashes in them.

Feature Use Case

Reduces bundle size and memory consumption. A quick experiment showed a reduction of 11Kb in my project.

Happy to work on a PR if this seems useful.

[alexander-akait]

Duplicate, webpack-contrib/css-loader#1029