Add a "Compose with Stackedit" button for the ZenDesk WYSIWYG editor, in case your company has chosen to use the WYSIWYG editor but you prefer writing comments with Markdown.

• Uses stackedit.js to show a useful user interface for editing Markdown and to convert that Markdown back into the HTML needed by the WYSIWYG editor.
• Uses turndown in order to convert any existing content back to Markdown, in case you started writing with the WYSIWYG editor and changed your mind later on (for example, if you want to add a lot of pre-formatted text).