Add Markdown to HTML rendering to item comments

Rendering done client side using Showdown: http://showdownjs.com
Live preview is shown below the form.
Only raw text is saved in the database (no HTML).

Note that no XSS filtering is done!
See https://github.com/showdownjs/showdown/wiki/Markdown's-XSS-Vulnerability-(and-how-to-mitigate-it)