Add functionality that can set key action

Thank you for this great product!
This PR extends key action (like keypress, keyup...) access. the functionality depends on [mousetrap](https://github.com/ccampbell/mousetrap).

angular-hotkeys is greate, but couldn't access Mousetrap's 3rd argument. As I want to create a keyboard shortcut that can be valid only while keypressing(holding down), add this functionality.

Here is an example:
```javascript
hotkeys.add({
  combo: 'w',
  description: 'description'
  callback: function() {
    /* callback */
  },
  action: 'keyup'
});
```