Add extension option to support Undefined as default value

[timriley]

If you include or extend dry-configurable like so:

extend Dry::Configurable(default_undefined: true)

Then Undefined will become the default value for all settings instead of nil (when no setting-specific default is provided).

When a setting has no value and Undefined is configured as the default, then the constructor will not be called.

See my initial spec to demonstrate all of this in a nutshell:

it "support Undefined as a default" do
  klass = Class.new do
    extend Dry::Configurable(default_undefined: true)

    setting :foo
    setting :bar, constructor: -> v { v.upcase }
    setting :baz, default: "baz", constructor: -> v { v.upcase }
  end

  expect(klass.config.foo).to be(Dry::Configurable::Undefined)
  expect(klass.config.bar).to be(Dry::Configurable::Undefined)
  expect(klass.config.baz).to eq "BAZ"
end

TBH, I think this PR is basically ready, apart from fleshing out that spec a little bit. I would love feedback at this point!

Resolves #108.

[solnic]

🚀 ❤️

[solnic]

Oh boy I am so happy to see this go away

[timriley]

Haha, you and me both

[flash-gordon]

LGTM 👍