Add a programmatic interface to manipulate nested fields

[gael-ian]

Currently the only way to add an item to a nested form is to trigger the click event on the add link.

Cocooned introduces referencing its own instance as a (jQuery) data attribute on the items container. Hence, it is theoretically possible to get that instance and call its add method to add a new item to the form. But the add method expect the add link and an event as arguments.

Cocooned should expose a clean interface to manipulate items in a nested form.

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Originaly suggested in nathanvda/cocoon#431