Add markdown injection

[samuelcolvin]

Systems commonly used markdown for convenience when defining templates for emails or user messages, however markdown is often seen as benign and not properly escaped. I propose you add a section on markdown injection.

Here are a few things to include:

**this is bold**
*unclosed bold
_this is italics_
_unclosed italics
[this is a link](http://example.com)
[this is a link](javascript:alert(25);)
![An image](http://example.com/image.jpg)
---
* bullet point

You could also add '# a titlebut that would currently break your formatting for comments inblns.txt`, not sure if you currently have a way around this?