For nested helpers: get the @ variables of the outer helper from the inner one

Following [this conversation](https://github.com/wycats/handlebars.js/issues/250#issuecomment-15836544), I think it would be very handy to be able, in nested helpers, to get the @ variables of the outer helper from the inner one.

    {{#each outer}}
        {{#each inner}}
            <!-- How to get the outer index here? -->
        {{/each}}
    {{/each}}

I am not sure what should be the syntax though...

    ../@index
    @../index

Thanks