Extract shared logic in Markdown parsers to trait

The following snippet is present in several files and should be merged into a trait

```php
if (isset($document->matter['title'])) {
  $this->title = $document->matter['title'];
} else {
  $this->title = $this->findTitleTag($document->body) ??
      Hyde::titleFromSlug($this->slug);
}
```` 