Micro optimizations to {{plugin-outlet}}.

* Don't create a `ContainerView` if there is only one connection.

* Don't create a virtual view if there are no connectors and the
  block form of the plugin isn't used.

app/assets/javascripts/discourse/helpers/plugin-outlet.js.es6

@@ -2,6 +2,8 @@
    A plugin outlet is an extension point for templates where other templates can
    be inserted by plugins.
 
+   ## Usage
+
    If you handlebars template has:
 
    ```handlebars
@@ -27,6 +29,16 @@
 
    And it will be wired up automatically.
 
+   ## The block form
+
+   If you use the block form of the outlet, its contents will be displayed
+   if no connectors are found. Example:
+
+   ```handlebars
+     {{#plugin-outlet "hello-world"}}
+       Nobody says hello :'(
+     {{/plugin-outlet}}
+   ```
 **/
 
 var _connectorCache;
@@ -77,13 +89,24 @@ export default function(connectionName, options) {
 
   var self = this;
   if (_connectorCache[connectionName]) {
-    var CustomContainerView = Ember.ContainerView.extend({
-      childViews: _connectorCache[connectionName].map(function(vc) {
-        return vc.create({context: self});
-      })
-    });
-    return Ember.Handlebars.helpers.view.call(this, CustomContainerView, options);
-  } else {
+    var view,
+        childViews = _connectorCache[connectionName].map(function(vc) {
+          return vc.create({ context: self });
+        });
+
+    // If there is more than one view, create a container. Otherwise
+    // just shove it in.
+    if (childViews.length > 1) {
+      view = Ember.ContainerView.extend({
+        childViews: childViews
+      });
+    } else {
+      view = childViews[0];
+    }
+    return Ember.Handlebars.helpers.view.call(this, view, options);
+  } else if (options.fn) {
+
+    // If a block is passed, render its content.
     return Ember.Handlebars.helpers.view.call(this,
               Ember.View.extend({
                 isVirtual: true,