Support nested elements

[lovelydinosaur]

Support nested navigations such as...

nav = [
    {title="Introduction", path="index.md"},
    {title="Tutorial", children=[
        {title="Creating a project", path="tutorial/new.md"},
        {title="Adding pages", path="tutorial/pages.md"},
        {title="Publishing your work", path="tutorial/publish.md"},
    ]},
    {title="Topics", children=[
        {title="Page layouts", path="topics/layouts.md"},
        {title="Typography", path="topics/typography.md"},
        {title="Color schemes", path="topics/schemes.md"},
    ]}
]