New Feature: Contenttypes can now specify their class.

Contenttypes can now specify the class to be used for records of that
contenttype. An extension can then define that class, overriding /
extending the default behaviour of \Bolt\Content.

For example:
```YAML
entries:
    name: Entries
    singular_name: Entry
    fields:
        title:
            type: text
            class: large
    (snip)
    class: \MyContent\Content
```

And in your extension, do:

```PHP
class Content extends \Bolt\Content
{

}
```