Load plugins from config; add minimal documentation on plugin system

It looks like there was is some basic support for plugins in Ghost, but the hooks to activate them were not loading from the config file (as the comments suggest they should). This fixes that.

In addition, there was essentially no documentation on the plugin system, nor were there any examples, so I threw together a minimal plugin and some documentation on how to use them (including updating the example configuration file with details on how to load it).