Feature Request: allow once modifier on custom event handlers

[humphd]

Having the ability to add modifiers for DOM events (#1088 )is really great, for example: on:click|once={handleClick}. Since this mechanism is now available, it might be useful to extend it to custom event handlers, such that you can do on:message|once={handleDispatch}. Obviously not all modifiers make sense in the context of a custom event. But for sure once would be useful.