Implement last author positional search

[aaccomazzi]

We know it's difficult, but the last author name (and last orcid, etc) position is significant for the life sciences.
Basically we need to implement the ability to say:

pos(author:"accomazzi, alberto", -1)

This should return all records where the search term has the last position.

All fields supporting positional searches should provide this functionality (e.g. orcid, institution, etc).

[aaccomazzi]

It would of course be nice to generalize this so we can search for a name in the last N positions of an author field:

pos(author:"accomazzi, a", -3)

and even:

pos(author:"accomazzi, a", -5, -1)

(which returns records where "accomazzi, a" appears in the last 5 positions).
As syntactic sugar, we could also adopt classic's notation with the $:

author:"$accomazzi, a"

(similar to what we do now with ^ for first author)

[JCRPaquin]

Partial implementation in this PR: #212