Return env if it already exists

Saves complicated workarounds like:
```javascript
var mode = process.env.NODE_ENV || 'development'
var env = (mode === 'production' || mode === 'development') ?
  browserify.settings[mode] : browserify.settings.env(mode)
env('transform', ['coffeeify'])
```
Now can just do:
```javascript
var env = browserify.settings.env(process.env.NODE_ENV || 'development')
env('transform', ['coffeeify'])
```