Add preventDefault and drag axis options

preventDefault
--------------

Returns or sets whether to prevent the browser's default behaviour in
response to pointer events. Can be set to

- `true` to always prevent
- `false` to never prevent
- `'auto'` to allow interact.js to try to guess what would be best
- `null` to set to the default (`'auto'`)

drag axis
---------

```javascript
interact.drag({ axis: 'x' || 'y' || 'xy' });
```

If set to 'x' or 'y', then the drag event sequence will only happen if
the direction of the first movement of the drag is along the given axis.

<br>
Closes #29