Handle multi-keystroke keybinding

Previously for a binding like `meta-x meta-y` you had to hold down the 
meta key. This makes it possible to release the meta key between
key presses.

spec/keymap-spec.coffee

@@ -203,6 +203,7 @@ describe "Keymap", ->
         describe "when a second keystroke added to the first to match a multi-stroke binding completely", ->
           it "triggers the event associated with the matched multi-stroke binding", ->
             expect(keymap.handleKeyEvent(keydownEvent('x', target: fragment[0], ctrlKey: true))).toBeFalsy()
+            expect(keymap.handleKeyEvent(keydownEvent('ctrl', target: fragment[0]))).toBeFalsy() # This simulates actual key event behavior
             expect(keymap.handleKeyEvent(keydownEvent('c', target: fragment[0], ctrlKey: true))).toBeFalsy()
 
             expect(quitHandler).toHaveBeenCalled()

src/keymap.coffee

@@ -177,7 +177,10 @@ class Keymap
   multiKeystrokeStringForEvent: (event) ->
     currentKeystroke = @keystrokeStringForEvent(event)
     if @queuedKeystrokes
-      @queuedKeystrokes + ' ' + currentKeystroke
+      if currentKeystroke in Modifiers
+        @queuedKeystrokes
+      else
+        @queuedKeystrokes + ' ' + currentKeystroke
     else
       currentKeystroke