Add the directive form of a group (`!group`)

Add the directive form of a group (`!group`)

This is to add some consistency regarding items which may contain other
items: I want all of them to have an available `!xyz` form.

Normand already has `!if` and `!repeat`, not it has `!group` (and the
alias `!g`).

The following two groups are equivalent:

    (
      aa bb cc
    ) * 5

    !group
      aa bb cc
    !end * 5

Change-Id: I00b2789c078c9b9216ab9b732bc3adeff39b0642
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>