Add autoprefixer toggle to config

[sebastiandedeyne]

There are two cases in which including autoprefixer by default gets in the way:

• When you're using a postcss plugin which already includes autoprefixer (like cssnext)
• or when you want to pass some extra configuration to autoprefixer

This PR adds a config toggle for autoprefixer, true by default so it's non-breaking, but it allows you to disable it if you want to deal with autoprefixer yourself in your project.