Move preprocessor dependencies to peerDependencies

[silverwind]

postcss-styl and postcss-scss dependencies could be moved to peerDependencies.

It would significantly lessen the number of modules this package depends on, while also eliminating the deprecated source-map-resolve dependency that npm warns about:

└─┬ [email protected]
  └─┬ [email protected]
    └─┬ [email protected]
      └─┬ [email protected]
        └── [email protected]