Added option to disable context menu and set disabled by default

As I'm a long time user of the Chrome extension, I didn't like the new context menu very much. 
So I've updated the options page to have a possibility to disable the context menu.
There is yet one bad thing, I didn't find a quick solution: When enabling the context menu, the browser needs to be restarted, so that the `menu.js` file will be run again. Perhaps there is a prettier solution for that?