Added Compatibility with "Association Field"

Shares the same logic as the basic "Selectbox Link Field", so just a one-liner :)
Allow ID-qualifier for associated entries

This change allows an ":id"-qualifier for selectbox-link-fields and association-fields.
Makes it possible to use the ID of an associated entry in addition to value and handle.

e.g.: `/page/{$category:id}/{$title}/`
Update Readme to include new {$fieldname:id} qualifier

Reflecting those two commits:

- https://github.com/twiro/link_preview/commit/9b6270141a8a258b6cbc6922201302fb8cd84300
- https://github.com/twiro/link_preview/commit/d4c9172ccff046ea66f3ebd8801a785635563619