update option to allow friendly field to update on subsequent saves

Completely up to you if you want to incorporate this, as it allows for circumventing the natural permanency of slugs but...

When not concerned with perma-link issues, and wanting slugs to match titles/names as models update - it would be ideal to allow the plugin to update the appropriate slug field.

This PR adds the following option (defaults to `false`) to toggle the slug overriding a previous slug on save.

```js
{ update: true/false }
```

Tests and docs updated as well to reflect the change...