Not necessary to create a new iframe for each call.

I have a project that makes many calls (some cases 100's) to `vm.runInNewContext`.

Each call to `vm.runInNewContext` incurs the creation and [insertion of a new `iframe`](https://github.com/browserify/vm-browserify/blob/master/index.js#L60).

This is not necessary, a single `iframe` can be cached, cleaned up after use and then re-used for subsequent calls to `vm.runInNewContext`.

I have a pr on the way.