Customizable loader class

[owais]

Webpack loader contains most of it's logic in the loader class. If it was possible to override this in django settings to use a custom implementation, a lot of use cases could be solved by subclassing and override them methods. This it the norm in django ecosystem anyway.

Example of issues that could be fixed by providing a customized version:

#136 override get_bundle, call super() and filter out the names of files.
#135

and many more requests I've come across over time.

PRs welcome! :)

[rhyneav]

Opened #210 to help solve this and provide flexibility to the package. Happy to discuss the implementation in the PR!