Restrict lookup for new item template to the closest item if exists

[gael-ian]

This should fix #61.

When a form uses Cocooned on multiple nested levels, the name data attribute of the template used to build new grand-child item does not change between child item (as all where built from the same template). If multiple child item are added to the form at the same time and each contains an add trigger to build new grand-child items, they will all share the same template name.

So far, lookup was done with document.querySelector but this will always return the first template with a matching name, the one of the first built child item. The options extractor will now look only inside the closest cocooned item to find the template and instantiate a builder with the correct template.

As a fallback, if no template is found, it will still look through the whole document so top-level Cocooned instance can still have their add triggers (and their associated template) anywhere.