support mutliple modifiers for key bindings

src/content/reducers/input.js

@@ -1,18 +1,18 @@
 import actions from 'content/actions';
 
 const defaultState = {
-  keys: ''
+  keys: []
 };
 
 export default function reducer(state = defaultState, action = {}) {
   switch (action.type) {
   case actions.INPUT_KEY_PRESS:
     return Object.assign({}, state, {
-      keys: state.keys + action.key
+      keys: state.keys.concat([action.key]),
     });
   case actions.INPUT_CLEAR_KEYS:
     return Object.assign({}, state, {
-      keys: '',
+      keys: [],
     });
   default:
     return state;