Plugin Support

- Use Role::Tiny to compose a class with roles via ->with_plugins
- `with_plugins` requires Role::Tiny, otherwise it's not a D::C dep
- Plugins MUST depend on Role::Tiny, and get composed into D::C classes to replace functionality.

The end goal is to let kd have a plugin that does sane zero-downtime restarts of PSGI stuff, which I then steal and use for all my own PSGI stuff.
