Functionality to add taxonomy terms as possible search results to the index

[Nomafin]

In some sites there is a need to include taxonomy terms as possible search results. We can achieve this by putting the terms in the search index as pseudo WP_Post objects thus allowing them to be included in the search results easily.

This has already been implemented as a custom feature in one of our own projects, so it can be fairly easily ported to the core from there.