Add an :initials merge tag modifier for Name fields

[zackkatz]

We added field settings for initials for Name fields in #1965, now we should have a merge tag modifier as well.

Then we could do something like this:

[gvlogic if="{Donate anonymously?:2}"]
    {Name:1.1:initials}, {Name:1.3:initials}
[else]
    {Name:1}
[/gvlogic]

Originally posted by @rafaehlers in #1965 (comment)