Bump showdown from 1.9.0 to 1.9.1

Bumps [showdown](https://github.com/showdownjs/showdown) from 1.9.0 to 1.9.1.
- [Release notes](https://github.com/showdownjs/showdown/releases)
- [Changelog](https://github.com/showdownjs/showdown/blob/1.9.1/CHANGELOG.md)
- [Commits](https://github.com/showdownjs/showdown/compare/1.9.0...1.9.1)

Signed-off-by: dependabot[bot] <support@github.com>

package.json

@@ -15,7 +15,7 @@
     "node-prismjs": "^0.1.2",
     "prismjs": "^1.16.0",
     "rss": "^1.2.2",
-    "showdown": "^1.9.0"
+    "showdown": "^1.9.1"
   },
   "devDependencies": {
     "coveralls": "^3.0.4",