Support nested layout: layout with its own layout

Test is added.

Layout for layout is useful, and is supported in Jekyll. I refactored the layout finding code out and call it again in `partial()` before the first layout is rendered.

All tests are passing.

The earlier layout explicit set to false fix is included too. The code change in `lib` is moved to `helpers/raw.js` with the whole layout finding logic. The test is still where it was, though.