The CSS and JS compressors can be turned off individually by specifying
either None or an empty string as the value for either
PIPELINE_CSS_COMPRESSOR or PIPELINE_JS_COMPRESSOR.

This is useful when you want to conditionally disable a compressor based
on what's available locally, or when a byproduct of a compiler is to
compress the content.

Signed-off-by: Timothée Peignier <timothee.peignier@tryphon.org>