Fix recreating context menus on startup.

  Some of error logs are:

  > Error during contextMenus.create: Cannot create item with duplicate id context-menu-item-0 sendRequest:22
  chromeHidden.handleResponse
  > Error during contextMenus.create: Cannot create item with duplicate id context-menu-item-1 sendRequest:22
  chromeHidden.handleResponse
  > Error during contextMenus.create: Cannot create item with duplicate id context-menu-item-2 sendRequest:22
  chromeHidden.handleResponse
  :
  :
Improve the timing of updating context menus on options.
Merge pull request #6 from toooooooby/fix-updating-contextmenu

Fix the timing of updating contextmenu after changing options.