Optionally increase heading level

## What did you do

I want to embed generated schemas into existing GitHub Pages where the heading levels should be increased to match the heading level structure of the GitHub Pages.

## What did you expect to happen

(this is a new feature)

## What happened

## What's your environment

* Operating System: macOS Mojave
* node.js version: 10.15.3

## Do you have example files:

n/a

No files or tests are named. Start by tracing the schema-to-Markdown heading-generation entry point and determine how options are exposed. Done means an optional heading-level offset can be supplied for embedded GitHub Pages content without changing the default generated headings.

Written by the indexing model from the issue text.