hide passwords by default

Hey folks, here's a stop gap patch to address the problem of accidentally
leaking passwords. It certainly isn't a full solution, but it might be helpful.

What do you think?

I'd appreciate pointers on how to write unit tests for this change.
The readme isn't too helpful:

```
$ grep -iA2 test README.md 
If you're working on the CLI and you can smoke-test your changes:

    $ bundle exec heroku
```

Who looks at this repo these days?

cc @fdr