Allow subscribers to unsubscribe on a per-object basis

This makes use of the new ES6 WeakMap feature, which allows for a hash
map that's keyed by object.

src/stdlib/subscriber.coffee

@@ -1,13 +1,28 @@
+_ = require 'underscore'
+
 module.exports =
   subscribe: (eventEmitter, eventName, callback) ->
     eventEmitter.on eventName, callback
     @subscriptions ?= []
-    @subscriptions.push(cancel: -> eventEmitter.off eventName, callback)
+    @subscriptionsByObject ?= new WeakMap
+    @subscriptionsByObject.set(eventEmitter, []) unless @subscriptionsByObject.has(eventEmitter)
+
+    subscription = cancel: -> eventEmitter.off eventName, callback
+    @subscriptions.push(subscription)
+    @subscriptionsByObject.get(eventEmitter).push(subscription)
 
   subscribeToCommand: (view, eventName, callback) ->
     view.command eventName, callback
     @subscriptions ?= []
     @subscriptions.push(cancel: -> view.off eventName, callback)
 
-  unsubscribe: ->
-    subscription.cancel() for subscription in @subscriptions ? []
+  unsubscribe: (object) ->
+    if object
+      for subscription in @subscriptionsByObject?.get(object) ? []
+        subscription.cancel()
+        _.remove(@subscriptions, subscription)
+      @subscriptionsByObject?.delete(object)
+    else
+      subscription.cancel() for subscription in @subscriptions ? []
+      @subscriptions = null
+      @subscriptionsByObject = null