Allow listeners to be removed via a Disposable returned from ::add

spec/command-registry-spec.coffee

@@ -77,3 +77,18 @@ describe "CommandRegistry", ->
 
     grandchild.dispatchEvent(new CustomEvent('command', bubbles: true))
     expect(calls).toEqual ['child-1']
+
+  it "allows listeners to be removed via a disposable returned by ::add", ->
+    calls = []
+
+    disposable1 = registry.add 'command', '.parent', -> calls.push('parent')
+    disposable2 = registry.add 'command', '.child', -> calls.push('child')
+
+    disposable1.dispose()
+    grandchild.dispatchEvent(new CustomEvent('command', bubbles: true))
+    expect(calls).toEqual ['child']
+
+    calls = []
+    disposable2.dispose()
+    grandchild.dispatchEvent(new CustomEvent('command', bubbles: true))
+    expect(calls).toEqual []

src/command-registry.coffee

@@ -1,3 +1,4 @@
+{Disposable} = require 'event-kit'
 {specificity} = require 'clear-cut'
 
 SequenceCount = 0
@@ -13,7 +14,15 @@ class CommandRegistry
       @rootNode.addEventListener(commandName, @dispatchCommand, true)
       @listenersByCommandName[commandName] = []
 
-    @listenersByCommandName[commandName].push(new CommandListener(selector, callback))
+    listener = new CommandListener(selector, callback)
+    listenersForCommand = @listenersByCommandName[commandName]
+    listenersForCommand.push(listener)
+
+    new Disposable =>
+      listenersForCommand.splice(listenersForCommand.indexOf(listener), 1)
+      if listenersForCommand.length is 0
+        delete @listenersByCommandName[commandName]
+        @rootNode.removeEventListener(commandName, @dispatchCommand, true)
 
   dispatchCommand: (event) =>
     propagationStopped = false