Added ability to continue event propagation while using data-event-click

Right now we have a situation where we want to open a link from a modal into a new tab (using `target="_blank"`), but also want to dismiss that modal and use a `data-event-click` on top. The current behaviour will fire the `data-event-click`, but prevent propagation of the event.

The proposed solution is to add an "allowedDefault" property to the node so that Batman knows not to kill the event (and allow the link to open in a new tab). I don't know if there is an existing precedence of `data-attrs` without arguments being used on nodes for Batman, so maybe this should switch to `data-allow-default`.

Open to suggestions and/or workarounds if this isn't elegant enough.

/cc @jbruton @nickjs @pushrax