Add support for inspecting and/or manipulating the reducer list

[DisposaBoy]

There are reducers like margoCodeCtl that are always enabled but are also configurable.
If it was able to inspect or manipulate the list, it would be able to look for the MargoCodeCtl reducer that provides its configuration instead of requiring MargoCodeCtl to update its config, thus making the code more complex because both reducers now need to care about potential races.

Allowing list manipulation would also simplify another variant of this use-case where we want a reducer to always be enabled, but only inserted if an instance isn't already inserted by the user.