Add new 'none' ENVIRONMENT

So in my project I'm manually downloading the WASM file, mainly so that I can do it in parallel with other large files, but also potentially so that I could add a download progress meter.

What would you think about adding a new ENVIRONMENT option of 'none' which would stop all shell type code from being emitted, would prevent the JS from trying to locate and download the WASM, would make the resulting code perfectly platform-agnostic, with no `require` calls, etc?

Things that would be slightly less optimal under this situation:

- `_emscripten_get_now` might be less accurate in Node
- something would need to be done for the TTY `get_char` in Node
- crypto in Node. Would it just have to abort? Fall back to Math.random? (I assume there are good reasons not to, as it currently aborts rather than falling back to Math.random.)