Core: Add multiple attribute to email method.

[RobJohnston]

Using @Sven-Mad's method. Fixes #1722.

[stale]

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Thank you for contributing :)

[RobJohnston]

I think this PR still has merit.

[Arkni]

Hi @RobJohnston,

Sorry for taking so long to review this PR. I only have one concern about this implementation. That no_multiple argument will always be true whether you are setting the rule using inline HTML or in JS.

What I would like to suggest is, we add support for passing an object to email method in JS like the following:

$("form").validate({
  rules: {
    field: {
      email: true | false | {
        multiple: true | false
      }
    }
  }
});

The no_multiple parameter will need to be changed to param (or another convenient name).

[RobJohnston]

Hi @Arkni . It's now my time to apologize to you for the delay! I've been away from front-end development for some time. I didn't quite understand your comment when I first read it and not sure if I do now.

From the W3, the multiple attribute is what they call a boolean attribute, but that doesn't mean that it can be set to "true" or "false". It must be multiple="multiple" or simply just multiple. To represent a false value, the attribute has to be omitted altogether. Does that change your suggestion where you wrote:

multiple: true | false