Added header item template feature

[kevinhoelscher]

This pull request adds an optional header-item-template attribute. It works much in the same way as the loading-template, only appearing if the attribute has a non-null value and the filtered list of items has a nonzero length. The idea is to be able to display results in a grid-like arrangement, where the data is complex enough that headers are needed for clarification.

[almothafar]

I reviewed your pull request, may you please add a demo for it as well to see a use case for it?

[kevinhoelscher]

Let me know if any further changes are needed.