Some new and improved settings

### New `context` setting for selector `Interactables`

```javascript
interact('ul.list > li', { context: element })
// or
interact('ul.list > li').context(element);
```

### Better `actionChecker`
>As for `actionChecker`, I think it would be good for me to change the way it works so that it calls the new function and passes to it the event object and the default action string. That would probably be a lot more useful and your workaround could be reduced to:

```javascript
interact('ul.list > li').actionChecker(function (event, action, interactable) {
    // return null if the event target is an input, textarea or anchor
    return (/^input$|^textarea$|^a$/i.test(event.target.nodeName)
    	    ? null
            : action);
});
```

### New `ignoreFrom` setting
If the target of the `mousedown`, `pointerdown` or `touchstart` event or any of it's parents match the given CSS selector or Element, no action is performed.

```javascript
interact(element, { ignoreFrom: document.getElementById('no-action') });
interact(element).ignoreFrom('input, textarea, a');
```

Closes #20