Refactor `spawn` plugin into separate module

[Marak]

The core spawning logic at https://github.com/Stackvana/microcule/blob/master/lib/plugins/spawn/index.js currently has two concerns coupled together: preparing the service options for spawning, and managing the spawned service.

microcule's spawning logic should only be concerned with preparing the service options. The actual spawning and managing of STDIO communications should be handled in a separate module.

To start, we can separate these concerns into separate files within this project. If it makes sense to then move the spawn logic itself to a separate package, we will.