Add option to set a custom display value on complete.

in Velocity UI, sometimes we don't want to automatically switch to the default CSS display value when transitioning in, so I've made a small edit in velocity.ui.js which now lets you supply an option 'display' to override the default value.

A use case would be, for instance, vertically centered text:
```css
div.center-vertically {
    display: none; /* must be set to 'flex' for vertical centering, 'block' will not center */
    flex-direction: column;
    justify-content: center;
}
```
Transitioning in while preserving vertical centricity:
```javascript
    $('.center-vertically').velocity('transition.expandIn', {duration: 500, display: 'flex'});
```