[boost] program_options suggestion

I really like program_options from Vladimir Prus. Thanks for adding bool_switch! One suggestion. We now have bool_switch, so a variable can default to off, and turn on with --option (without having to say --option=true). How about begin able to say --no-option to turn it off? This would make the interface consistent with gnu standards. If it's not too difficult it would make a good addition.