Adds the option to include an ID attribute on headings

[danclien]

I'm not sure this belongs in markdown or not.

The use case of enabling msAddHeadingId is for generating URLs that can jump directly to a heading.

For example, given a document that may look like this:

# My Report
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Duis eleifend in dui sed interdum. Sed eget sapien sit amet nibh lacinia tristique. Proin malesuada, ipsum nec congue commodo, metus nulla sagittis arcu, sit amet interdum urna libero nec nunc.

## Introduction
In in consequat lacus. Sed quis ipsum non mi ultricies volutpat in id enim. Quisque non imperdiet nunc, ut rhoncus odio. Fusce ipsum est, ultrices non libero in, ornare ultricies mauris. 

## Summary
Phasellus mattis ultrices erat in ultrices. Nunc eget accumsan eros, sed placerat dui. Vestibulum congue scelerisque mauris sed fringilla. Vestibulum mollis vitae felis sed volutpat. Morbi sollicitudin venenatis vulputate.

The generated HTML for ## Summary would be

<h2 id="summary">Summary</h2>

Now, I can jump directly to the Summary by using the http://mywebsite.com/#summary URL.

Comments or suggestions welcomed.

[snoyberg]

Looks great, well done! Clear documentation and test suites included :)