Bump showdown from 1.9.1 to 2.0.0 (#977)

Bumps [showdown](https://github.com/showdownjs/showdown) from 1.9.1 to 2.0.0.
- [Release notes](https://github.com/showdownjs/showdown/releases)
- [Changelog](https://github.com/showdownjs/showdown/blob/2.0.0/CHANGELOG.md)
- [Commits](showdownjs/showdown@1.9.1...2.0.0)

---
updated-dependencies:
- dependency-name: showdown
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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