Ensure options are deep extended

Was there a reason why deep extend was removed ?
https://github.com/SassDoc/sassdoc-theme-default/blob/develop/index.js#L37

So the user willing to override only one specific option in its `.sassdocrc` or `--config` as in:
```js
"display": {
  "watermark": false
}
```

is getting an Error:
```
[ERROR] TypeError: Cannot call method 'indexOf' of undefined
at /[...]/sassdoc-theme-default/node_modules/sassdoc-extras/src/display.js:9:48
```