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:

└─┬ eslint-plugin-vue-scoped-css@2.8.0
  └─┬ postcss-styl@0.12.3
    └─┬ stylus@0.57.0
      └─┬ css@3.0.0
        └── source-map-resolve@0.6.0