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/parse-command.js

@@ -0,0 +1,32 @@
+export function parseCommand(cmd) {
+    const extraSpacesStrippedCommand = cmd.replace(/\s{2,}/g, ' ');
+    const splitCommand = extraSpacesStrippedCommand.split(/\s+(?![^[]*]|[^<]*>)/);
+    const bregex = /\.*[\][<>]/g;
+    const firstCommand = splitCommand.shift();
+    if (!firstCommand)
+        throw new Error(`No command found in: ${cmd}`);
+    const parsedCommand = {
+        cmd: firstCommand.replace(bregex, ''),
+        demanded: [],
+        optional: [],
+    };
+    splitCommand.forEach((cmd, i) => {
+        let variadic = false;
+        cmd = cmd.replace(/\s/g, '');
+        if (/\.+[\]>]/.test(cmd) && i === splitCommand.length - 1)
+            variadic = true;
+        if (/^\[/.test(cmd)) {
+            parsedCommand.optional.push({
+                cmd: cmd.replace(bregex, '').split('|'),
+                variadic,
+            });
+        }
+        else {
+            parsedCommand.demanded.push({
+                cmd: cmd.replace(bregex, '').split('|'),
+                variadic,
+            });
+        }
+    });
+    return parsedCommand;
+}