Toggle changes in Markdown preview diff #3711

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Feature

To be able to hide/show the diff in Markdown change previews.

For example, when editing a Markdown file on GitHub.com, you can preview your changes: https://github.com/sindresorhus/refined-github/edit/master/readme.md

Use-case

I often edit markdown files in the GitHub UI because it's quick and easy, but when the changes are complex, it becomes hard to see the final output because the diff clutters the preview.

In these cases, I would love to be able to just see the final output instead of what I removed/modified.

<img width="1112" alt="Screen Shot 2020-11-10 at 2 22 05 PM" src="https://user-images.githubusercontent.com/1075694/98723305-2f294b00-2360-11eb-9716-5cc93364b09c.png">