Fix Sub Lists

[Steppschuh]

Adding an UnorderedList as new UnorderedListItem to an existing UnorderedList should create a sub-list.

    @Test
    public void example() throws Exception {
        List<Object> items = Arrays.asList(
                "Item 1",
                "Item 2",
                new UnorderedList<>(Arrays.asList(
                        "Item 2.1",
                        "Item 2.2",
                        "Item 2.3"
                )),
                "Item 3"
        );
        System.out.println(new UnorderedList<>(items));
    }

Should print:

• Item 1
• Item 2
  • Item 2.1
  • Item 2.2
  • Item 2.3
• Item 3