Added option to show selected items

[zaru]

Proposal.

Add option to display items selected by multiple attributes. This option allows for flexibility in the way it looks.

For example, in CSS, it can be displayed comma-separated.

.nice-select.has-multiple {
  .multiple-options {
    display: block;
    text-overflow: ellipsis;
    white-space: nowrap;
    overflow: hidden;
  }
  .current {
    &:before {
      content: ' , ';
    }
    &:first-of-type {
      &:before {
        content: '';
      }
    }
  }
}