yoda: Add ability to declare options with optional argument

The 'optarg' keyword means, that an argument is optional and the
rest of the string is considered to denote the argument value for
that case.

Signed-off-by: Pavel Emelyanov <xemul@sacred.ru>
yoda: The default and optarg strings may contain "-s

Actually commas are not required, but it might be useful to
add/emphasize spaces/tabs in the value with them.

Signed-off-by: Pavel Emelyanov <xemul@sacred.ru>