Advanced context menu

When creating a Package for Atom, I realized I needed submenus in Context menus.

This mod allows Context menus to accept the same JSON/CSON structure as regular menus.
Example:
```
'context-menu':
  '.workspace': [
    { 'type' : 'separator' }
    {
      'label' : 'Test submenu'
      'submenu': [
        { 'label': 'test', 'command': 'test-submenu:toggle' }
      ]
    }
    { 'type' : 'separator' }
    { 'label' : 'Lala', 'command': 'test-submenu:toggle'}
  ]
```

Feel free to comment. This is my first time ever touching Coffeescript.

Tested on Ubuntu 14.04.

![Screenshot](https://cloud.githubusercontent.com/assets/2339213/2934564/b22c2902-d7d4-11e3-9044-7b8f4728a5bd.png)