Update `with_subelements` to deeper located sub-lists

Before, `with_subelements` could only handle that the nested list was one element 'deep' compared to the first argument.

Now, `with_subelements` accepts the second parameter in the form of `foo.bar.baz`, which will get the list at `foo.bar.baz` instead of complaining that the literal key `foo.bar.baz` doesn't exist.