How to change option HTML elements' attribute of ChoiceField?

[Taka Hi]

I want to change option HTML attributes per option.

example:

```

<select>

  <option data-xxx="data-of-a" value="value_of_a">1. a</option>

  <option data-xxx="data-of-b" value="value_of_b">2. b</option>

</select>

```

I think that it is needed to pass extra-data from ChoiceField to ChoiceWidget Select.

How can I do this?