Add a table of contents to some sites that use markdown like reddit wikis, rentry or github.io pages

Example sites to test this with:
- https://rentry.co/FMHY
- https://www.reddit.com/r/piracy/wiki/index/
- https://wiki.dbzer0.com/piracy/
- https://saidit.net/s/freemediaheckyeah/wiki/index
- https://luong-komorebi.github.io/Awesome-Linux-Software/
- https://madaidans-insecurities.github.io/guides/linux-hardening.html
- https://anonymousplanet.org/guide.html

For GitHub or GitHub Gists pages, use this script instead: https://greasyfork.org/scripts/18344