ENH: Added doNotChangeFileExt to prevent changes from .jsx to .js

We use karma along with `karma-commonjs-plus` to run our unit tests. The default behavior of changing the fileName from .jsx to .js was breaking our code since the code have require calls like this

`require('./some/component.jsx');`

which were not longer valid after the transformation.

This pull request adds a property to the configuration object of this preprocessor. I'm using the preprocessor now from my fork. But it would be nice to use it from the official source, once you merge this pull request.

Regards