Enforce file naming conventions for plugins

[papercast-dev]

Enfore a file structure for Python packages from each plugin that looks like:

plugin_dir
├── processors.py
├── publishers.py
├── subscribers.py
└── types.py

Benefits:

• Easier stub generation for #9
• Consistent init.py across plugins
• Consistency/structure might be a good thing for plugin contributors

Cons:

• Slightly harder to open files for developers working with several plugins at once