Add support for nested ordered list

[bjtho08]

Currently, only unordered lists are supported. I would like support for ordered lists.

The end result would be something like this:

% The code block below
```toc
title: # Table of Contents
style: nestedOrderedList
```
% results in the following toc
# Table of Contents
1. First heading
2. Second Heading
    1. First Sub-Heading
    2. Second Sub-heading
3. Third Heading

% given these headings in the document
# First heading
# Second Heading
## First Sub-Heading
## Second Sub-heading
# Third Heading

[johansatge]

Supported in 1.4.0, thanks!