Support `<optgroup>`

[kAlvaro]

Currently, selects are expected to be flat. There's no JSON syntax or code to handle <optgroup> tags.

The label attribute is mandatory so that should simplify the syntax.

In case it helps, here's a quick idea (probably absurd because I'm actually not very familiar with the plug-in):

["": "None"],
[
    {
        Asia": [
            ["cn","China"],
            ["jp","Japan"]
        ],
    }
],
[
    {
        "Europe": [
            ["fr", "France"]
            ["es", "Spain"],
        ],
    }
],
["zz": "Other"],
["--": "Don't know"]

[Aeon]

+1!!!

[federicodv]

+1 !

[kAlvaro]

I remind you you can add a Thumbs up icon to any entry in issue tracker, there's no need to flood the ticket with "+1" comments.