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/.bin/showdown

@@ -0,0 +1,16 @@
+#!/bin/sh
+basedir=$(dirname "$(echo "$0" | sed -e 's,\\,/,g')")
+
+case `uname` in
+    *CYGWIN*|*MINGW*|*MSYS*)
+        if command -v cygpath > /dev/null 2>&1; then
+            basedir=`cygpath -w "$basedir"`
+        fi
+    ;;
+esac
+
+if [ -x "$basedir/node" ]; then
+  exec "$basedir/node"  "$basedir/../showdown/bin/showdown.js" "$@"
+else 
+  exec node  "$basedir/../showdown/bin/showdown.js" "$@"
+fi