allow include by section?

[fundthmcalculus]

The goal of this PR is to enable including only a specific section of a file, as specified by the header.

[fundthmcalculus]

@cmacmackin thoughts?

[ZedThree]

@fundthmcalculus Please could you give an example of this?

[BlakeFreer]

This should include all content up to the next heading of equal or greater importance.

For example, given the following document called readme.md

Lorem Ipsum

Usage

Installation

Install with pip install lorem-ipsum

Importing

import lorem_ipsum

Issues

Contact me at example@email.com

Then you could include the "Usage" section (up to and excluding ## Issues) with

{!readme.md!section=usage}

Rather than needing to know the exact line numbers. This syntax plays nicely with the existing Markdown syntax where you can link to this section heading with #usage