pass indentation config to css.stringify

Helps the library comply with Jest's serializer's indent configuration,
which in turn fixes styled-components#355

Author: fritz-c

src/styleSheetSerializer.js

@@ -70,15 +70,15 @@ const getAtRules = (ast, filter) =>
       return acc.concat(atRule);
     }, []);
 
-const getStyle = (classNames) => {
+const getStyle = (classNames, config = {}) => {
   const ast = getCSS();
   const filter = filterRules(classNames);
   const rules = ast.stylesheet.rules.filter(filter);
   const atRules = getAtRules(ast, filter);
 
   ast.stylesheet.rules = rules.concat(atRules);
 
-  return css.stringify(ast);
+  return css.stringify(ast, { indent: config.indent });
 };
 
 const getClassNamesFromSelectorsByHashes = (classNames, hashes) => {
@@ -149,7 +149,7 @@ module.exports = {
     classNames = filterClassNames(classNames, hashes);
     unreferencedClassNames = filterUnreferencedClassNames(unreferencedClassNames, hashes);
 
-    const style = getStyle(classNames);
+    const style = getStyle(classNames, config);
     const classNamesToReplace = getClassNamesFromSelectorsByHashes(classNames, hashes);
     const code = printer(val, config, indentation, depth, refs);