Does not work with Immutable.js

This is not a bug with `isomorphic-webpack` per-se, though it is an issue that I have ran into while working on a PR to add `isomorphic-webpack` to https://github.com/mxstbr/react-boilerplate.

The relevant links:

* http://stackoverflow.com/questions/41598098/how-to-make-immutabe-js-work-inside-vm
* https://github.com/facebook/immutable-js/issues/1039

Short description:

Immutable.js `fromJS` does not correctly recognise a plain object. As a result, it returns the input object without converting it to a map.

This is just the first issue I have come across. I am sure there will be related issues.

I will document a workaround as soon as I know how to approach this issue.