Fieldset Repeater: need to be able to track deletion of existing fieldsets

Scenario: I come back to edit an invoice and want to destroy an existing associated line item. Simply removing the fields won't have an impact on a framework like rails.

The rails convention to handle this is:

```html
<input type="hidden" name="invoice[line_items_attributes][{{{id}}}][_destroy]" value="" />
```

But this needs to be flexible, so it might need to be driven by a `<template />`.