Add a plugin feature

[davorg]

I think it works like this:

• Plugins are called App::Aphra::Plugin::*
• Looks for a plugins key in site.yml this is a list
• For each value in that list, load the module App::Aphra::Plugin::<value>
• Call the mandatory register_plugin() subroutine in the plugin module, passing it the App::Aphra object
• We also need a role that forces plugins to have a register_plugin() method