lib: Add nexthop-group cli

lib: Add nexthop-group cli

Add a nexthop-group cli:

nexthop-group NAME
  nexthop A
  nexthop B
  nexthop C
!

This will allow interested parties to hook into the cli for
nexthops.  Users can add callback functions for add/delete
of a nexthop group as well as add/delete of each individual
nexthop.

Future work( PBR and static routes ) will take advantage
of this.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>