Add tundra.list.string:first to return the first item in a given
list; closes #87
Add tundra.list.string:last to return the last item from a given
list; closes #88
Add tundra.list.document:first for returning the first item in a
given list; closes #85
Add tundra.list.document:last for returning the last item from a
given list; closes #86