yoda: Add "for" keyword

This one denotes, that the option in question is optional for
anything. After the kwd there should go a string describing what
the option is optional for.

If the expression is false and the option is provided, an warning
is reported.

Signed-off-by: Andrey Vagin <avagin@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@sacred.ru>