Add event tracking to window.GOVUK.analytics

app/assets/javascripts/errorTracking.js sent
events to `window.ga`.

This extends the API of `window.GOVUK.Analytics`
to include support for sending events so all
calls to `window.ga` can use it instead of direct
access.

This use of `window.ga` was missed from the
initial work on `window.GOVUK.Anaytics`.

app/assets/javascripts/analytics/analytics.js

@@ -4,7 +4,7 @@
   window.GOVUK = window.GOVUK || {};
 
   // Stripped-down wrapper for Google Analytics, based on:
-  // https://github.com/alphagov/static/blob/master/app/assets/javascripts/analytics_toolkit/analytics.js
+  // https://github.com/alphagov/static/blob/master/doc/analytics.md
   const Analytics = function (config) {
     window.ga('create', config.trackingId, config.cookieDomain);
 
@@ -34,6 +34,29 @@
 
   };
 
+  // https://developers.google.com/analytics/devguides/collection/analyticsjs/events
+  Analytics.prototype.trackEvent = function (category, action, options) {
+
+    options = options || {};
+
+    var evt = {
+      eventCategory: category,
+      eventAction: action
+    };
+
+    if (options.label) {
+      evt.eventLabel = options.label;
+      delete options.label;
+    }
+
+    if (typeof options === 'object') {
+      $.extend(evt, options);
+    }
+
+    window.ga('send', 'event', evt);
+
+  };
+
   window.GOVUK.Analytics = Analytics;
 
 })(window);