[js] @:expose enum to js

I can't find a way to expose an enum to js, and I see that recently someone else posted on gitter with the same problem.

I'm currently using a hackish solution to achieve it (see [this discussion](https://groups.google.com/forum/#!searchin/haxelang/expose/haxelang/ZAZtlc2VqHs/mx6hxS77CgAJ) on the mailing list), but I'm wondering if there's a standard way to do it or if it has been intentionally left out.