manualStart for actions

```javascript
interact(target)
  .draggable({
    manualStart: true
  })
  .on('hold', function (event) {
    var interaction = event.interaction;

    if (!interaction.interacting()) {
      interaction.start({ name: 'drag' },
                        event.interactable,
                        event.currentTarget);
    }
  });
```

`Interactable#start` takes 3 arguments:

 1. An object describing the action. It needs a `name` property which is a string. For resize it can also have an `axis` property.
 2. The `Interactable` to work with
 3. The Element to target

The pointer must be down and there must be no running interaction for `Interactable#start` to work.

Closes #114