message: Allow passing sourceMap options to PostCSS
  PostCSS accepts `boolean | SourceMapOptions` as `options.map`.
  This slightly changes the default behavior, as "map: true" will default to inline.
  We can change it to `options.sourceMap === true ? {inline: false} : options.sourceMap`, but I'm not sure is worth keeping the extra weirdness. WDYT?
diff: @@ -69,7 +69,7 @@ export default function (userOptions?: Partial<Options>) {
  async function postCss(file: Page) {
  const from = site.src(file.src.path + file.src.ext);
  const to = site.dest(file.dest.path + file.dest.ext);
  - const map = options.sourceMap ? { inline: false } : undefined;
  + const map = options.sourceMap;
  // Process the code with PostCSS
  const result = await runner.process(file.content!, { from, to, map });
  