Add support for starting/stopping a single component

[oubiwann]

This will, of course, need to do the same for its dependencies ...

See:

• com.stuartsierra.component/dependencies (link)
• com.stuartsierra.component/dependency-graph (link)
• com.stuartsierra.component/update-system-reverse (the 'stop' phase of restart; link)
• com.stuartsierra.component/update-syste (the 'start' phase of restart; link)