Add markdown_escape setting

This setting allows to escape HTML in markdown rendering or not.
The goal behind it is to avoid XSS issue in shared instances.

More info:

  * the setting is set to true by default
  * it is set to false for anyone who already have the plugin enabled
  (avoid breaking existing entries)
  * improve the HTML sanitization when the setting is set to false - but don't consider it XSS proof
  * mention the setting in the plugin README