Add a way to prevent the Module from being exported globally.

In our application, we stuff emscripten into a closure in pre- and post-
JS files, and export our interface without exposing the Module that we
use to implement some of it.
do these comparisons work as strings? i would guess you need to quote `noexport` there.

how about just checking if `EXPORT_NAME` is empty, as opposed to a magical value (where someone might have a typo)?

We'll need testing for this option.