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.

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