Allow multiple value selection for 'select' elements

Here is the patch to use.

Hi @euk,
I have used these configurations to add style class for a select dropdown

attributes:
  container_class:
    label: 'Container class(es)'
    description: 'CSS class for the menu list item (<li>). Separate multiple classes by space.'
  class:
    label: 'Link class(es)'
    description: 'CSS class for the link (<a href>). Separate multiple classes by space.'
  target:
    label: 'Link target'
    description: 'Choose how the link should open.'
    options:
      _blank: 'New window (_blank)'
      _self: 'Same window (_self)'
    default_value: ''
  style_class:
    label: 'Style class'
    description: 'Choose style classes for this link.'
    options:
      btn: 'Button'
      primary: 'Primary'
      large: 'Large'
    multiple: true

and after applying your MR I was able to select more than one.
Attaching ss for reference.
So moving it to RTBC.

Thanks,
Arun