Use add/removeFile methods to normalize integration with other component plugins

This keeps things clean for other component plugins. For instance I created a `component-autoprefixer` plugin that will autoprefix the css. This needs to happen after stylus transpiling is done. `component-autoprefixer` will check `pkg.config.styles` as it should, but it assumes that plugins like `component-stylus-plugin` are working with `addFile` and `removeFile` transparently.

Make sense?