Fix locale for partials using `context.stack.tail`

The `formatDate` helper works on main page but failed in nested partials.  Inspecting the context revealed nested partials use `context.stack.tail...` instead of `context.stack.head...`
Fix locale if helper nested within two sections

I found that if helper is nested within section nested within another that an additional `.tail` is added in context object.  This should cover most use cases but likely best to build some code to "walk" the context object and find the `locality`.
Merge pull request #2 from mikesparr/master

Add support for helper nested within partial with multiple sections