Bump showdown from 1.9.1 to 2.1.0

Bumps [showdown](https://github.com/showdownjs/showdown) from 1.9.1 to 2.1.0.
- [Release notes](https://github.com/showdownjs/showdown/releases)
- [Changelog](https://github.com/showdownjs/showdown/blob/master/CHANGELOG.md)
- [Commits](showdownjs/showdown@1.9.1...2.1.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

package-lock.json

@@ -39,7 +39,7 @@
     "npm-watch": "0.11.0",
     "pug": "3.0.1",
     "sass-loader": "6.0.7",
-    "showdown": "^1.9.1",
+    "showdown": "^2.1.0",
     "style-loader": "0.23.1",
     "uglify-js": "3.14.1",
     "uikit": "3.0.0-rc.26",