Add support for internal links

[ediordna]

Add support for internal links

At the moment, links to headings inside the file aren't working, see following example:

## Getting Started

Lorem Ipsum 

## Development

For xyz see section [Getting Started](#getting-started)

I would expect sphinx-mdinclude to generate a working internal link to the heading Getting Started. Instead, just the plain text inside of the [] brackets is displayed in the Sphinx Docs.