update showndown to 1.9.1

this has openLinksInNewWindow which is smart enough that internal links
open in same tab and external links open in new tab

src/js/client.js

@@ -89,10 +89,10 @@ angular.module('Application').filter('prettyDiskSize', function () {
 
 angular.module('Application').filter('markdown2html', function () {
     var converter = new showdown.Converter({
-        extensions: [ 'targetblank' ],
         simplifiedAutoLink: true,
         strikethrough: true,
-        tables: true
+        tables: true,
+        openLinksInNewWindow: true
     });
 
     // without this cache, the code runs into some infinite loop (https://github.com/angular/angular.js/issues/3980)