Add mailmap entry for Chris Vittal

I use both Chris and Christopher in git configs and other places.

.mailmap

@@ -51,6 +51,7 @@ Carol Willing <carolcode@willingconsulting.com>
 Chris C Cerami <chrisccerami@users.noreply.github.com> Chris C Cerami <chrisccerami@gmail.com>
 Chris Pressey <cpressey@gmail.com>
 Chris Thorn <chris@thorn.co> Chris Thorn <thorn@thoughtbot.com>
+Chris Vittal <christopher.vittal@gmail.com> Christopher Vittal <christopher.vittal@gmail.com>
 Clark Gaebel <cg.wowus.cg@gmail.com> <cgaebel@mozilla.com>
 Clinton Ryan <clint.ryan3@gmail.com>
 Corey Richardson <corey@octayn.net> Elaine "See More" Nemo <corey@octayn.net>