Ignore commented links in SUMMARY.md

[pelted]

It would be very helpful to be able to comment out chapter links in my SUMMARY.md file so that they are not processed. Currently we have to delete lines then git revert the file before committing changes to the summary.

When working with a large document with many chapters they may have processing issues or take a long time to process, it is beneficial to "turn off" some chapters to speed to temporarily ignore issue.

Expected behavior:

* [Introduction](README.md)
* [1. Chapter 1](./chapters/chapter_01.md)
* [2. Chapter 2](./chapters/chapter_02.md)
<!--
* [3. Chapter 3](./chapters/chapter_03.md)
* [4. Chapter 4](./chapters/chapter_04.md)
-->

• Introduction
• 1. Chapter 1
• 2. Chapter 2

Actual Result

• Introduction
• 1. Chapter 1
• 2. Chapter 2
• 3. Chapter 3
• 4. Chapter 4