Move postcss to peerDependencies

[stalniy]

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