Add compress skip patterns from settings

[PetrDlouhy]

This combines both approaches to skipping files mentioned in #279:

• Skip based on regexp patterns defined by WHITENOISE_SKIP_REGEX settings (--skip-regexp parameter)
• Make the compressor class interchangable by WHITENOISE_COMPRESSOR_CLASS settings (--compressor-class parameter)

@LucidDan @merwok Please take a look at this. If everything is OK, I will add documentation for the new parameters/settings.

[adamchainz]

I'd prefer not to add the regex setting when users can achieve the same in a few lines of code with a custom Compressor class, with more flexibility.

(Also your example for the regex strays a bit from the recommended setup. Sass/Less files should not be in the directory that Whitenoise serves - only compiled CSS files. So this makes me pause to think if the idea of skipping some files for compression is really valid, or it could be solved with better guidance for project structure.)

[PetrDlouhy]

@adamchainz My complete regex list is:

WHITENOISE_SKIP_REGEXP = (
    r".*/src/",
    r".*\.scss$",
    r".*\.less$",
    r".*\.webm$",
    r".*map$",
    r".*\.md$",
    r"wagtailyoast.*",
    r"wagtailadmin.*",
    r"drf-yasg.*",
)

and while some of the scss might be possible to put outside static files dirs, others might be quite difficult. For example, django-scribbler contains static/scribbler/less/scribbler.less file. Also the folders like wagtailadmin are quite large and those files are needed only for admins, so the compression is not that essential.
Most of the files I want to exclude are coming from external applications, where it would be difficult to change project structure.

I think, that skipping compression of files by pattern is quite clear use-case and having to override the Compressor class would make unnecessary complexity in settings files.

But anyway, I can make separate MRs for those two things, so you can merge the WHITENOISE_COMPRESSOR_CLASS code.
I will have to look at my project structure more deeply and think through if it couldn't be in fact be reorganized or something.