Add cloneable setting option

[timriley]

This is an evolution from the approach in #100. With this change, settings may be specified with a cloneable option, e.g.

setting :component_dirs, Configuration::ComponentDirs.new, cloneable: true

This makes it possible to provide “rich” config values that carry their own configuration interface.

In the example above, ComponentDirs could provide its own API for adding component dirs and configuring aspects of their behavior at the same time. By being passed to the setting along with cloneable: true, dry-configurable will ensure the setting's values are cloned along with the setting at all the appropriate times (see Setting#initialize_copy).

As the example here suggests, this change is in service of dry-system providing a new ”rich” component_dirs setting, providing an API like this:

# Inside a Dry::System::Container subclass
configure do |config|
  config.component_dirs.add("path/to/dir") do |dir|
    dir.auto_register = true
    dir.add_to_load_path = true
    # etc.
  end
end

[timriley]

FYI, I've tested this in dry-rb/dry-system#155 and it's working nicely there.