Adding custom attributes for option tags within SelectField and SelectMultipleField

Currently you do not allow the user to create html attributes on `<option>` tags within `SelectField` and `SelectMultipleField`. For example if a user wanted `<option data-id="3">` it is not possible to create that with your current `Select` classes. This is something that I personally have needed and others have requested. 

To use this new feature, for example:

     groups_form.people.choices = [(p.id,p.full_name,{'data-role':p.role}) for p in group.people.all()]

If that third element is left out, everything stays the same. 
The code above resulted in the following HTML for me:

     <option data-role="admin" selected="" value="38">Payton Small</option>