Support multiline variables

Many heroku addons set multiline variables (e.g. ssl certs). This PR writes the .env as

```
KEY="VALUE"
KEY2="MULTILINE\nVALUE"
KEY3="VALUE\"WITH\" QUOTES"
```
and uses YAML to parse the `\n` and `\"` from an existing config.

[dotenv](https://github.com/bkeepers/dotenv) supports parsing these.