Add (picrin optional).

The library provides some macros to handle optional arguments. The
macros are compatible with them in Chicken, Gauche, etc.

Note that Chicken raises an error when any excess arguments are
provided, while Gauche ignores them. (picrin optional) is aligned to
Gauche.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>