Allow uncritical configuration

[nachogarcia]

Latest critical version has a target option that allows to save the remaining css (uncritical) to a different file

  target: {
    css: 'critical.css',
    html: 'index-critical.html',
    uncritical: 'uncritical.css',
  },

It would be great if we could use that functionality to be able to use this library to extract critical css without duplicity if we're not inyecting it in HTML.

[bezoerb]

@nachogarcia: Thanks for reporting this.
I don't have free time to work on this at the moment. If you like you can open a PR for this to speed things up.