Use height of items to compute heightDelta

## Use case

When using a list with a fixed height set to show an exact number of suggestions, it creates a much better effect if the list scrolls by exactly the height of one suggestion when you use the cursor keys to scroll through.

### Examples

**Before: Using fixed value of `heightDelta: 25` last element is cut off**

![demo_2](https://cloud.githubusercontent.com/assets/104138/4400593/c078501a-4482-11e4-8f43-714b37606a20.gif)

**After: Using automatically calculated value of `heightDelta: 43`**

![demo](https://cloud.githubusercontent.com/assets/104138/4400597/c7dcb54e-4482-11e4-8389-f343dbc99016.gif)

Thanks for an awesome plugin by the way! :thumbsup: