message: feat(dropdown): add option to let event propagate on toggle
  Issue valor-software/ngx-bootstrap#1217 was fixed but re-introduce
  recently in development branch (commit 04cab1eb6f42ad7a6456c34a0ec54f9bd37fd83a).
  Let's add an option to authorize propagation of click event on toggle
  for users that need it.
diff: @@ -3,6 +3,7 @@ import {
  ElementRef,
  HostBinding,
  HostListener,
  + Input,
  OnDestroy
  } from '@angular/core';
  import { Subscription } from 'rxjs/Subscription';
  @@ -17,6 +18,11 @@ import { BsDropdownState } from './bs-dropdown.state';
  }
  })
  export class BsDropdownToggleDirective implements OnDestroy {
  + /**
  + * Set it to false to let click event propagate.
  + */
  + @Input() suppressClick: boolean = true;
  +
  @HostBinding('attr.disabled') isDisabled: boolean = null;
  // @HostBinding('class.active')
  @@ -41,7 +47,10 @@ export class BsDropdownToggleDirective implements OnDestroy {
  @HostListener('click', ['$event'])
  onClick(event: MouseEvent): void {
  + if (this.suppressClick) {
  event.stopPropagation();
  + }
  +
  if (this.isDisabled) {
  return;
  }
  