Customize stylesheet entry points

[jmckible]

In order to support additional stylesheets or non-standard naming, this change will expose the build path via a configuration hash. I've started with a pattern of:

config.dartsass.stylesheets = { 'application.scss' => 'appliction.css' }

In actual code, this ends up looking like this in our application:

# config/initializers/dartsass.rb
Rails.application.config.dartsass.stylesheets = {
  'app/index.sass'   => 'app.css',
  'embed/index.sass' => 'embed.css',
  'site/index.sass'  => 'site.css'
}

I'm open to a different convention. I could see using more implicit magic (extracting the output naming from the input file (or directory, in the case of index.sass/scss). But I don't have a sense of where this is aiming (perhaps some coupling with propshaft?), so this felt like the simplest option.

[dhh]

I think this is good 👍. Although. Might want to consider renaming the config variable.