Doesn't work with `ember-cli-postcss`

[boris-petrov]

Extracted from #1. This comment (and the ones around it) have some explanations and clues.

@webark the best way would be to fix the underlying issue (be it in ember-cli-styles or in ember-cli-postcss). Until then, we have to use the workaround in the linked comment. However, in my own configuration of ember-cli-postcss I have also a filter step which includes plugins like autoprefixer, cssnano, etc. How do I do the same with emberCliStylesOptions?

[webark]

Adding a "filter" (or post-process) option in ember-cli-styles-preprocessor would be nice. Might want to change the name of the module to ember-cli-styles-processor or something like that if we do that. This can be something I work though.