integrate envvar

[envvar][1] is a tiny package for dealing with environment variables in a more rigorous manner.

There's so much imprecision in an expression such as the following:

```javascript
process.env.CI_BUILD_NUMBER || 0
```

The result of the expression will be a string or a number depending on whether `CI_BUILD_NUMBER` is set and, if so, whether its value is `''`. There's no validation, so the result may be `'XXX'`.

I realize this imprecision is *usually* okay in practice, so I won't be offended if you decide not to merge this pull request. :)


[1]: https://github.com/plaid/envvar