[BUG] --no-[parameterName] doesn't allow setting to null for some typeDefs

[speige]

The purpose of "--no-myParam" is to allow setting a parameter to null instead of undefined, because "-myParam null" would set it to the string value 'null' instead of the javascript value null.

However, this feature is buggy for some typeDefs.

if typeDef is String
"--no-myParam" the parse() function sets it to false instead of null
However, clean() internally calls validateString() which changes it to 'false'

so, "--no-myParam" has no purpose in the case of typeDef String because i could have used "-myParam false" instead.

I haven't tested to see if the other typeDefs have similar issues, but it's possible.