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.