Add `afterLoad` plugin interface

docs/customization/plugin-api.md

@@ -19,7 +19,8 @@ A plugin return value may contain any of these keys, where `myStateKey` is a nam
   },
   components: {},
   wrapComponents: {},
-  fn: {}
+  afterLoad: (system) => {}
+  fn: {},
 }
 ```
 
@@ -363,7 +364,35 @@ const MyWrapComponentPlugin = function(system) {
 }
 ```
 
+##### `afterLoad`
 
+The `afterLoad` plugin method allows you to get a reference to the system after your plugin has been registered with the system.
+
+This interface is used in the core code to attach methods that are driven by bound selectors or actions directly to the system.
+
+```javascript
+const MyMethodProvidingPlugin = function() {
+  return {
+    afterLoad(system) {
+      // at this point in time, your actions have been bound into the system
+      // so you can do things with them
+      system.myMethod = system.exampleActions.updateFavoriteColor
+    },
+    statePlugins: {
+      example: {
+        actions: {
+          updateFavoriteColor: (str) => {
+            return {
+              type: "EXAMPLE_SET_FAV_COLOR",
+              payload: str
+            }
+          }
+        }
+      }
+    }
+  }
+}
+```
 
 ##### fn
 

src/core/system.js

@@ -68,6 +68,14 @@ export default class Store {
     if(rebuild) {
       this.buildSystem()
     }
+    
+    if(Array.isArray(plugins)) {
+      plugins.forEach(plugin => {
+        if(plugin.afterLoad) {
+          plugin.afterLoad(this.getSystem())
+        }
+      })
+    }
   }
 
   buildSystem(buildReducer=true) {

test/core/system/system.js

@@ -683,4 +683,31 @@ describe("bound system", function(){
     })
   })
 
+  describe("afterLoad", function() {
+    it("should call an plugin's `afterLoad` method after the plugin is loaded", function() {
+      // Given
+      const system = new System({
+        plugins: [
+          {
+            afterLoad(system) {
+              system.wow = system.dogeSelectors.wow
+            },
+            statePlugins: {
+              doge: {
+                selectors: {
+                  wow: () => (system) => {
+                    return "so selective"
+                  }
+                }
+              }
+            }
+          }
+        ]
+      })
+
+      // When
+      var res = system.getSystem().wow()
+      expect(res).toEqual("so selective")
+    })
+  })
 })