fix: use indented sass syntax when lang="sass"

Fix vuejs#103

Author: znck

src/style/scss.js

@@ -9,6 +9,7 @@ export default function (style, options) {
         omitSourceMapUrl: true,
         sourceMap: true,
         outFile: style.id,
+        indentedSyntax: style.lang === 'sass',
         ...options.scss
     })