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
Here's an alternative that defines the passing cases to reduce the complexity of the condition:

```js
            if (node.declarations.length === 1 ||
                !inStrictMode && isForStatement ||
                exceptUndefined && !hasDefinedVariables) {
                return;
            }

            errors.add('Multiple var declaration', node.loc.start);
```