add validateIfExists method

[mocheng]

In my practical programming context, there are such kind of code

function some_action(req, res) {
   var validr = new Validr(req.body);
   if (req.body.phone_num) { 
       validr.validate('phone_num', 'phone number  should be valid').isNumeric();
   }

}

Some arguments, suck as phone_num in above code is optional. So, we only need to validate it if it exists. However, having if statement to check its existence introduce increased cyclomatic complexity.

If there an validateIfExists function to do validation only given field exists or empty. That would be great.

[mocheng]

Or else, we can have add method ``ignoreEmpty&ignoreUndefined`property to `validr` .