N°5621 Move showdown 2.0.0-alpha1 to NPM

Current file header is : /*! showdown v 2.0.0-alpha1 - 25-09-2018 */

Official 2.0.0-alpha doesn't match neither on content nor date :(
Considering date we would be at 1.8.7, but it is also quite different (and has vulnerabilities)
In consequence switching to 2.0.0.

node_modules/yargs/build/lib/utils/which-module.js

@@ -0,0 +1,10 @@
+export default function whichModule(exported) {
+    if (typeof require === 'undefined')
+        return null;
+    for (let i = 0, files = Object.keys(require.cache), mod; i < files.length; i++) {
+        mod = require.cache[files[i]];
+        if (mod.exports === exported)
+            return mod;
+    }
+    return null;
+}