Rule for validating whitespace.

@param rule The validation rule.
@param value The value of the field on the source object.
@param source The source object being validated.
@param errors An array of errors that this rule may add
validation errors to.
@param options The validation options.
@param options.messages The validation messages.
function whitespace(rule, value, source, errors, options) {
  if (/^\s+$/.test(value) || value === '') {
    errors.push(util.format(options.messages.whitespace, rule.fullField));
  }
}