Generate header IDs for secondary headers and deconflict them within a file.

Hi Rich,

This utility change enables us to generate header IDs for secondary headers (h2, h3, ...) that are unique within each LDN article.

As a reminder, during conversion to HTML by the Markdown Converter, an anchor with the same ID is created just before the header.

For example, Markdown source line ...

`## What is an App?'

... is converted to ...

'<a id="what-is-an-app"></a><h2>What is an App?</h2>'