added support to build the release for any mix environment

As described in #60, building releases for different mix environments would be an important feature for us to have. To get to know this tool it a bit better I gave it a shot.

As suggested by @ericmj I changed it to read the MIX_ENV variable

```
MIX_ENV=staging mix release
```

I tested it for our deployment and it seems to work the way we need it work (I haven't tested appups though). Let me know if you want me to apply additional changes.

If you worked on that already or dislike my solution don't hesitate to simply ignore this PR ;-)