Add an option to enable `rewritePolyfills`

[teppeis]

It's enabled by default in CLI, but disabled in API (CompilerOptions). https://github.com/google/closure-compiler/blob/4d7a626b9a9796fbce17577c23d92ef2e80a2c5b/src/com/google/javascript/jscomp/CompilerOptions.java#L915

So polyfill is not available with plovr now. I eager it!