add failing test for nested layout (layout with layout)
add support for nested layout (layout with layout)

1. layout with `layout` defined in _data.json will be rendered
inside that layout after including its own yielded content
2. layout's layout has to be explicitly defined, so nearest
layout will not be used in mistake. Also, layout with itself as
layout will be ignored the second time
3. layout finding code is yanked from `render()` into `helpers` as
a standalone function. `partial()` calls `findLayout()` to find
an explicit layout for the to-be-rendered layout itself.