Wishlist: add a `--env` option to `%%script` to set environment variables

It would be nice to be able to set environment variables when invoking an external program/script. It can currently be done with command line games, eg: on most linux flavours you could do:

```
%%script env myScriptVar=someValue ruby
...
```

but it would seem more natural for the capability to be in `%%script`, not the least because it could be platform agnostic.

One potential approach would be to allow arbitrary `Popen` args (as a dict perhaps?).

ps: the `env` command also allows unsetting environment variables - another useful option, but perhaps less important..