Pull request making items available as property

Hi Karl-Heinz,

I've added a property name "iterator.items" to the AbstractIteratorMojo.
Please have a look and maybe pull it.
I found it quite convenient to be able to specify the items, e.g. to build just a specific version of an artifact rather than the whole set of items.

Fixes khmarbaise/iterator-maven-plugin#4.

Regards,

Florian