refactor less/stylus/sass support

[willkg]

There are a lot of preprocessors out there and currently support for them is hard-coded into jingo-minify. This really needs a better solution.

At a minimum, it should get abstracted out. Maybe allow for plugin support for additional things so we don't have to maintain them all, too.

[willkg]

SASS and Stylus support is pretty similar. The LESS support has an additional LESS_PREPROCESS thing.

In refactoring we should figure out whether to keep that functionality or ditch it. I'm inclined to ditch it for reduced complexity.

[cvan]

yeah, I'd be a fan of ditching it. we're removing LESS_PREPROCESS from zamboni any day. (it's too slow for development.)

[pmclanahan]

django-compressor provides base classes for "filters" which you can subclass and thereby provide your own processing for whatever you want. I like this approach a lot. We create classes for which ever we want out-of-the-box support for, accept pull-requests for others, and allow people to easily define their own. We could auto-import a specific module name in apps (a la helpers.py for jingo), maybe minifiers.py, and auto-register classes to file extensions via metaclass. It's a common django pattern, and should be a lot easier on us.

from django.conf import settings

from jingo_minify.minifiers import BaseCompiler


class StylusCompiler(BaseCompiler):
    type = 'styl'
    bin = getattr(settings, 'STYLUS_BIN', 'stylus')
    command = '{bin} < {infile} > {outfile}'

That along with some hook methods for overriding could get us pretty far. The ones django-compressor has are fairly complex at this point, but I'm not sure ours would need to be.