Error message formatting

I added an easy way to format error messages with the values used. Shouldn't have made any backwards incompatible changes.

It works like:

    var error_messages = {between: '{{0}} is not between {{1}} and {{2}}'};
    var izObj = iz(5, error_messages).between(100, 200);
    console.log(izObj.errors); // -> ['5 is not between 100 and 200']