Add support for mount

[bn-darindouglass-zz]

Context

Currently formatting-stack supports Component and Integrant hooks. mount is another popular state-management library that would be easily supported.

Task

Add new formatting-stack.mount namespace that adds mount-up/on-up hooks for formatting code.

Acceptance criteria

Mount is pluggable.

Additional resources

The mount-up library will work easily for adding this hook. Something small like should suffice:

(speced/defn start [^map? this]
  ...)

(mu/on-up :formatting start :after)