generate CSS sourcemaps

[Rich-Harris]

Changes the first argument to the css callback to be an object with code and map properties, and a write method for convenience that generates a CSS file with an accompanying sourcemap:

// rollup.config.js
export default {
  entry: 'src/main.js',
  dest: 'public/bundle.js',
  format: 'iife',
  plugins: [
    svelte({
      cascade: false,
      css: css => {
        css.write('public/bundle.css'); // generates bundle.css and bundle.css.map
      }
    })
  ]
};

The write method sets the map.file property correctly and makes the sources in the sourcemap relative to the output file, so it's recommended over handling the code and the map directly (though that's totally possible).

Attempting to use css directly in a string will invoke the toString method, which prints a deprecation warning before returning css.code.

[Rich-Harris]

Fixes #14, but depends on sveltejs/svelte#716