[WIP] Automatic Section Numbering in Gitbook Sidebar

Note that this plugin is a work in progress and won't behave as expected.

This plugin will take a SUMMARY.md that you've already written and add section numbers in the generated Gitbook's sidebar.

For example, given this SUMMARY.md file:

# Table of Contents
* [Read Me](/README.md)
* [Introduction](/introduction/README.md)
  * [Motivation](/introduction/Motivation.md)
  * [Getting Started](/introduction/Getting_Started.md)
* [Tutorial](/tutorial/README.md)
  * [Simple Widgets](/tutorial/Simple_Widgets.md)
  * [Interact](/tutorial/Interact.md)

This plugin will generate:

# Table of Contents
* [1. Read Me](/README.md)
* [2. Introduction](/introduction/README.md)
  * [2.1 Motivation](/introduction/Motivation.md)
  * [2.2 Getting Started](/introduction/Getting_Started.md)
* [3. Tutorial](/tutorial/README.md)
  * [3.1 Simple Widgets](/tutorial/Simple_Widgets.md)
  * [3.2 Interact](/tutorial/Interact.md)