Make autofocus textbox module

This commit adds a very small Javascript module to autofocus a textbox on page
load. It should only be used once per page.

It also adds a parameter to the textbox macro which adds the attribute to
trigger autofocus.

app/assets/javascripts/autofocus.js

@@ -0,0 +1,12 @@
+(function(Modules) {
+  "use strict";
+
+  Modules.Autofocus = function() {
+    this.start = function(component) {
+
+      $('input, textarea, select', component).eq(0).trigger('focus');
+
+    };
+  };
+
+})(window.GOVUK.Modules);