Move postcss to peerDependencies

`postcss-sprites` is a plugin for `postcss` thus should include `postcss` as `peerDependency` and not as `depedency` in package.json.

Currently I have several postcss plugins and all of them use different version of postcss because nobody uses `peerDependencies` :) 

Please read this article https://nodejs.org/en/blog/npm/peer-dependencies/ for more details. I'll create a PR soon