data-sly-list and data-sly-repeat iteration control

[raducotescu]

Both data-sly-list and data-sly-repeat should be enhanced to provide iteration control through the following additional expression options:

• begin - iteration begins at the item located at the specified index; first item of the collection has index 0
• step - iteration will only process every step items of the collection, starting with the first one
• end - iteration ends at the item located at the specified index (inclusive)

Example usage:

<!--/* Iterate the collections starting from the element at index 5 (6th element), jumping every 5 elements, until index 100 */-->
<ul data-sly-list="${collection @ begin = 5, step = 5, end = 100}">
    <li>${item}</li>
</ul>
<section data-sly-repeat="${collection @ begin = 5, step = 5, end = 100}">${item}</section>

[raducotescu]

Added to spec in e9a4d27 and 4267b77.