EXTENSIBILITY: add filter api to inject hooks in raw templates

TODO: decide with @eviltrout,
decide if registerUnboundOverride makes more sense

app/assets/javascripts/discourse/helpers/topic-link.js.es6

@@ -1,9 +1,14 @@
 import registerUnbound from 'discourse/helpers/register-unbound';
+import { runFilters } from 'discourse/lib/filter';
 
 registerUnbound('topic-link', function(topic) {
   var title = topic.get('fancyTitle');
   var url = topic.linked_post_number ? topic.urlForPostNumber(topic.linked_post_number) : topic.get('lastUnreadUrl');
 
   var extraClass = topic.get('last_read_post_number') === topic.get('highest_post_number') ? " visited" : "";
-  return new Handlebars.SafeString("<a href='" + url + "' class='title" + extraClass + "'>" + title + "</a>");
+  var string = "<a href='" + url + "' class='title" + extraClass + "'>" + title + "</a>";
+
+  string = runFilters('topic-link', string, topic);
+
+  return new Handlebars.SafeString(string);
 });

app/assets/javascripts/discourse/lib/filter.js.es6

@@ -0,0 +1,24 @@
+var filters = {};
+
+// use filter API to register a callback from a plugin
+const filter = function(name, fn) {
+  var current = filters[name] = filters[name] || [];
+  current.push(fn);
+};
+
+const runFilters = function(name, val) {
+  const current = filters[name];
+  if (current) {
+
+    var args = Array.prototype.slice.call(arguments, 1);
+
+    for(var i = 0; i < current.length; i++) {
+      val = current[i].apply(this, args);
+    }
+  }
+
+  return val;
+};
+
+export { runFilters };
+export default filter;