List tag: Add array operations - push, pop, shift, unshift

It would be useful for the `List` tag to have more array operations.

- `push` - Append item(s) to end of list
- `pop` - Remove the last item
- `shift` - Remove the first item
- `unshift` - Prepend item(s) to start of list

They could support the short syntax (attribute with no value); and long syntax with attribute name and value, like `action=push`.
