Add a way to set the "none" value for options

[SamWhited]

Right now if an option is set to "none" it's just not written. This is fine normally (the defaults get used), but if you set the option, then set it back to none, it doesn't get written and the option isn't reverted to the default.

Adding an extra argument (none_value?) to the option constructor would do the trick (also a separate method like param_writer when we're doing things manually).

This is needed for #46 in particular.