Option to add custom actions to autocomplete

[OFouda]

We would like to be able to extend the autocomplete with additional actions.

In our case we provide users with an option to use a list of keywords within his query (written in codemirror). If the starts typing a keyword we want to show all possible matches with a button below the autocomplete suggestions (at the end of the dropdown) to create a definition for a new one. Ideally, we would be able to simply append any DOM element to the autocomplete dropdown.

Something like this screenshot here would be great

[marijnh]

I've considered this, but it seems too much of a special case to include support for it in the library.