postRender callback for array item container

Wanted a way to modify styling of an added array item, but didn't see a postRender callback mechanism for items.

[Update] I moved the call to postRender after the toolbar has been rendered so you have more flexibility in updating the content/styling.

Since the item toolbar is just prepended to the container, this postRender callback can be used to move it to a different location.  I don't see a way to affect the toolbar location using templates.  Maybe the fieldSetItemContainer template should have an empty div for the toolbar to be placed rather than hardcoding it in a particular location.  The same could be argued for the itemLabel template who's placement is also hardcoded to be prepended to the container element.

Regardless, I think this postRender callback is a useful feature.