Make redactor fields previewable

[MoritzLost]

Redactor fields can't be selected for custom sources because the redactor field class doesn't implement the PreviewableFieldInterface. It would be great if the plugin would support this – there are many cases having a preview of a redactor field in an element index would be useful. I assume this shouldn't be too difficult. For my use-case a simple the implementation that strips HTML and truncates the text to a predefined length would suffice.