Switch Showdown to fork

fixes #2312

- showdown fork understands more html tags
Update README.md
Update CONTRIBUTING.md
Merge pull request #2451 from ErisDS/showdown-fork

Switch Showdown to fork
Removing typography extension

issue #2312

- The typography extension is still interfering in HTML blocks, reference style links and other bits and pieces it probably shouldn't be :(
- We'll add it back when it's ready.
Merge pull request #2452 from ErisDS/typography

Removing typography extension
Improving the showdown extensions

fixes #2381

- renamed the ghost extensions
- added new html tests