Add cut/copy/paste built-in context menu items

Closes https://github.com/atom/atom/issues/1638

This change allows built-in context menu commands to be defined in Atom's menu config files (096255f) and adds the core copy/cut/paste commands to the context menu (776a8e9).

The built-in context menu commands are defined in the same way as they are in packages -- there's a `context-menu` key, and commands scoped to a selector. 

Here's the menu in action:

![copy](https://cloud.githubusercontent.com/assets/38924/4059313/487fc4fe-2de3-11e4-8a47-699219c332ab.gif)

cc @kevinsawicki for :eyes: