Add querytag for alterable realname query.

The patch.

Thank you for your reply, I appreciate the feedback.

What kind of issues could it possibly cause?

With this queryTag you can alter the query if necessary for your specific case, You might want to join a field to check on something and makes this an small change which has less impact on this module compared to a field instance setting. I have added the field instance as meta data, so it's possible to alter the query for a specific field, entity_type or bundle.

In my case I wanted users to select a user entity reference without:
* user 1
* blocked users

I have named it specific so it wouldn't collide with an existing tag and therefore possible create issues.
A field setting would be too limited in my opinion, because it could be any property you'd want to filter on, as in my case also user 1.

My first approach was overriding the #autocomplete_path which is overridden by realname with a custom callback (due to this fact which was almost a copy of the realname's callback). I thought this would be an cleaner way to do this, with less impact.

Entityreference module which provide the default autocomplete also provide a tag: entityreference. So I don't see why this is not an easy and correct way to do this. It might benefit users wanting to use realname, but don't need the forced autocomplete widget callback or want to use it, but with some minor adjustments.

Although it might be better to call the tag: realname_autocomplete so it's unique enough not to collide with future query tags on other realname queries.