add markdown plugin

This pull request adds the new Markdown plugin. That plugin will need to be published to npm first.

https://github.com/fedwiki/wiki-plugin-markdown

We've taken our guidance from the github guide. Unimplemented features or under-implemented features could appear or improve at a later date.

https://guides.github.com/features/mastering-markdown/

The About page describes the plugin as follows.

![image](https://cloud.githubusercontent.com/assets/12127/5425898/4aa7f068-82e2-11e4-9ebd-e2fa33437961.png)