Changable compressor class

[PetrDlouhy]

Fixes #279.

[dulacp]

@adamchainz could this be merged once rebase? I think this feature would be great to give users flexibility.
@PetrDlouhy, if yes, could you rebase your PR?

[adamchainz]

I haven't thought too hard about whether this is a good solution. It's a large API area to expose to users.

The PR is also far from ready: there are no tests, the documentation needs expanding significantly to describe how to subclass Compressor and what methods can be customized, there are no non-django docs, and there's no changelog entry.

I would also like to see some evidence of experimentation with alternative approaches, even just monkey-patching the current version of Whitenoise to see how easy it is to skip files that way.

[PetrDlouhy]

@adamchainz I have rebased to current main and dealt with the settings problem.

Have you made some decision whether you want this in the project or not? I would like to work on the tests and documentation, but I don't want to waste my time if it doesn't eventually get merged.

I would also like to see some evidence of experimentation with alternative approaches, even just monkey-patching the current version of Whitenoise to see how easy it is to skip files that way.

I am not sure how to deal with this. Of course it would be possible with monkey-patching, but it would be possible, but it would be very ugly.
Different approach might be to allow to only override the should_compress() method.

[PetrDlouhy]

@adamchainz I have updated this, wrote tests and added some more documentation.
What do you think? What is left to do in order to merge this?