publish(event, data = {}) {
        let self = this;
        
        // There's no event to publish to, so bail out
        if(!self.events.hasOwnProperty(event)) {
            return [];
        }
        
        // Get each subscription and call its callback with the passed data
        return self.events[event].map(callback => callback(data));
    }
If the passed event has callbacks attached to it, loop through each one
and call it

@param {string} event
@param {object} [data={}]
@returns {array} The callbacks for this event, or an empty array if no event exits
@memberof PubSub