disallowMultipleVarDecl: add exception for undefined variable declaratio...

...ns

Rather messy as boolean logic could be simpler?
Not sure about the option name `exceptUndefined` either.

#451
This is duplicative of https://github.com/jscs-dev/node-jscs/pull/749/files#diff-bbab9dfea97994901be20fd462a701c0R42. Can we remove one of them?