"Group" changes

The "group" command now takes an optional parameter, "add" or "remove", that
could be useful in crowded situations.

For example:

> group 3.horse add
(Add 3.horse to your group. If 3.horse is already a group member, do nothing.)

> group 3.horse remove
(Remove 3.horse from your group. If it is not a group member, do nothing.)

If you do not specify add/remove, the "group" command works as a toggle,
as usual: it groups the target if it is not in the group, it ungroups the
target otherwise.