Allow defining custom attachments without sgid

[tonysm]

We need an extension point to allow creating custom attachments that don't require having an sgid... Possible syntax:

// In AppServiceProvider#boot
RichTextLaravel::withCustomAttachable(function (DOMElement $node) {
  if ($attachable = OpenGraph::fromNode($node)) {
    return $attachable;
  }
});

These "factories" would be used in the AttachableFactory::fromNode() here.