Define a plugin architecture for simple definition of extensions

[lmprice]

If I'm a developer that wants to add monitoring support for a specific application, I don't necessarily want to fork the project in order to do so. There should be a simple way of including extension functionality without impacting the core functionality or having to hack it in.

Preferably, we would define the required directories, files, and layout for each for defining a plugin and it could be self-contained.