Feature: Select form control multiselect

[MitchellVermaning]

Hello,

Thank you for this great library.

I added to a new form control the attribute multiple on the HTML element select (form/select.js). When I set two options to selected, only the last selected option will be highlighted.

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HTML preview:

HTML rendered the following code:

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To reproduce this problem:

1. Copy the code from form/select.js.
2. Add multiple: true to attrs. Like:
   
3. Add the following options:

[
    {
      label: "test1",
      value: "test1",
      selected: true
    }, {
      label: "test2",
      value: "test2",
      selected: true
    }, {
      label: "test3",
      value: "test3",
    }
]

4. When you rendered the form, only option test2 will be selected. While the HTML code has selected test1 and test2.

Do you have any idea how I can resolve this problem? Thank you!