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/set-blocking.js

@@ -0,0 +1,12 @@
+export default function setBlocking(blocking) {
+    if (typeof process === 'undefined')
+        return;
+    [process.stdout, process.stderr].forEach(_stream => {
+        const stream = _stream;
+        if (stream._handle &&
+            stream.isTTY &&
+            typeof stream._handle.setBlocking === 'function') {
+            stream._handle.setBlocking(blocking);
+        }
+    });
+}