Support template defined groups in `ngx-select`

[Hypercubed]

trafficstars

Support template defined groups in ngx-select similar to native optgroup.

Example:

<ngx-select [groupBy]="'type'">
  <ngx-optgroup [name]="'IOS'">
    <ngx-select-option [name]="'Breach'" [value]="'breach'"></ngx-select-option>
    <ngx-select-option [name]="'DDOS'" [value]="'ddos'"></ngx-select-option>
  </ngx-optgroup>
  <ngx-optgroup [name]="'MOX'">
    <ngx-select-option [name]="'Physical'" [value]="'Physical'"></ngx-select-option>
  </ngx-optgroup>
</ngx-select>