Transition on show and hide.

How about having a transition duration on .show and .hide? I find this easier on the eye.
```javascript
    tip.hide = function() {
      var nodel = d3.select(node)
      nodel.transition()
        .duration(transitionDuration)
        .style({ opacity: 0, 'pointer-events': 'none' });
        
      return tip
    }
```

200ms works well:

```javascript
    var tip = d3.tip()
      .attr('class', 'd3-tip')
      .html(function(d) { return '<span>' + d.total + '</span>' + ' entries' })
      .offset([-12, 0])
      .transitionDuration(200)
```

Great library, by the way. Thank you!