Add preFormatted function to Revision

Added preFormatted function to Revision base model to allow for easier display of revision histories. Still get the revision history as before:

$article = Article::find($id);
$history = $article->revisionHistory;

but now instead of formatted the display manually each time:
@foreach($account->revisionHistory as $history )
    <li>{{ $history->userResponsible()->first_name }} changed {{ $history->fieldName() }} from {{ $history->oldValue() }} to {{ $history->newValue() }}</li>
@endforeach

you can call:
@foreach($account->revisionHistory as $history )
    return $history->preFormatted();
@endforeach