Send cookies on requests to another domain

[gualopezb]

I've been using the ahoy.js library to create ahoy events on a rails server on a different domain which has the ahoy gem configured. The events are being created successfully. However, I need to send the cookies on the request, because the server gets some extra data from that.
The above requires the following option to be set on the ajax request:

xhrFields: {
  withCredentials: true
}

Is there an option on ahoy.js to make this work?

I'm setting ahoy config in this way:

ahoy.configure({
  urlPrefix: 'http://otherdomain.com'
});

[gualopezb]

@ankane This PR solves the issue: #19

The usage would be:

ahoy.configure({
  crossDomain: true
});

By default the crossDomain option is set to false.