[Dropdown] Add possibility to add header items during js initialization

[prudho]

Closes issue Semantic-Org/Semantic-UI#6420.

Allow to add header items to dropdown during js initialization, using this syntax:

$('.ui.dropdown').dropdown({
    values: [
      {
        name     : 'Filter by tag',
        type     : 'header'
        // Will be displayed as header
      },
      {
        name     : 'Important',
        value    : 'important',
        type     : 'item'
        // Will be displayed as item
      },
      {
        name     : 'Announcement',
        value    : 'announcement'
        // Will be displayed as item
      }
    ]
  });

Works too with API requests.