feat: Allow handling Open URL actions in user code

[gyoshev]

Allows handling of Open URL actions in user code, replacing the default window.location.assign code.

Triggers a preventable openUrl event that receives the URL to open. User code looks like:

Leanplum.on('openUrl', function(e) {
  // prevent default open
  e.preventDefault();

  // open in new tab, or use in client-side router, etc.
  window.open(e.url, '_blank');
});