Replace Mitchelf/Markdown with Parsedown

fixes #5518

library/vendor/Parsedown/.gitignore

@@ -0,0 +1,3 @@
+.DS_Store
+.idea
+nbproject

library/vendor/Parsedown/LICENSE.txt

@@ -0,0 +1,20 @@
+The MIT License (MIT)
+
+Copyright (c) 2013 Emanuil Rusev, erusev.com
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software is furnished to do so,
+subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

library/vendor/Parsedown/README.md

@@ -0,0 +1,31 @@
+## Parsedown
+
+Better [Markdown](http://en.wikipedia.org/wiki/Markdown) parser for PHP.
+
+***
+
+[ [demo](http://parsedown.org/demo) ] [ [tests](http://parsedown.org/tests/) ]
+
+***
+
+### Features
+
+* [fast](http://parsedown.org/speed)
+* [consistent](http://parsedown.org/consistency)
+* [GitHub Flavored](https://help.github.com/articles/github-flavored-markdown)
+* friendly to international input
+* [tested](https://travis-ci.org/erusev/parsedown) in PHP 5.2, 5.3, 5.4, 5.5 and [hhvm](http://www.hhvm.com/)
+
+### Installation
+
+Include `Parsedown.php` or install [the composer package](https://packagist.org/packages/erusev/parsedown).
+
+### Example
+
+```php
+$text = 'Hello _Parsedown_!';
+
+$result = Parsedown::instance()->parse($text);
+
+echo $result; # prints: <p>Hello <em>Parsedown</em>!</p>
+```

library/vendor/Parsedown/VERSION.txt

@@ -0,0 +1 @@
+0.8.3