Add option to export scan results as "markdown"

## Short Description

As a user, I would like a command line option (e.g. `--markdown`) so that I can export the scan results in "markdown" format. This would enable the user to upload these markdown pages to a github wiki.

## Possible Labels

- new feature

## Select Category

- [x] Enhancement
- [ ] Add License/Copyright
- [ ] Scan Feature
- [ ] Packaging
- [ ] Documentation
- [ ] Expand Support
- [ ] Other

## **Describe the Update**

At the moment, there is already support for exporting scan results to JSON, CSV, HTML, and to a customize template. Being that markdown is one of the most popular formats used by programmers to document things in their repositories and wikis, it would be nice to have a quick way of doing this rather than have to create a custom template. I have tried converting HTML to markdown with a few different python libraries and it never converts properly mainly because the usage of the `` tag in the HTML format and it also leaves CSS styling in the page.

## **How This Feature will help you/your organization**

This would allow me to commit the scan results to a wiki page (which renders markdown), for example, to github's wikis.

## **Possible Solution/Implementation Details**

1. Add a new command line option such as `--markdown FILE` that will format the scan results accordingly; or,
2. Modify the HTML formatting so that it can better converted by (python) libraries like `markdownify` or `html2markdown`.

## **Example/Links if Any**

```
docker run -v $PWD/:/output scancode-toolkit -clpeui --processes 4 --markdown /output/$FILENAME /output
```

## **Can you help with this Feature**