Improve default behavior

I ran into https://github.com/hughsk/envify/issues/7 again today, which made me wonder if we might improve the default behavior or `envify`.

Instead of having to choose between `require('envify')` and `require('envify/custom')` this behavior might be more intuitive.

``` js
var envify = require('envify');

// detects the process.env.NODE_ENV variable
browserify()
.transform(envify())

// manually override the environment
browserify()
.transform(envify({env: 'custom'});
```

Let me know what you think, if you approve I'll PR the changes.