Toastr multi icon implementation

[tudornico]

The current implementation of the toastr HTML code https://github.com/akveo/nebular/tree/master/src/playground/with-layout/toastr does not provide the inclusion of multiple icons inside the toastr ( for example a close button X on the top) .
Future implementation of this issue could have specific handler action on clicks inside the toasr pop-up.