Add support for optgroup in select components

[luxigo]

This PR adds basic support for the <optgroup> tag within the Select component.
This allows for better categorization and organization of long option lists.

{
    "tag": "select",
    "label": "Select it",
    "options": [
      "Other",
      [ "optgroup", " Category 1" ],
      "Option 1",
      "Option 2",
      ["/optgroup"],
...
}

[SergeyMosin]

@luxigo If you want a key to unlock the contributor features please email me at sergey [at] srgdev.com