Update Initialiser to set run! method on target gem

- Initialiser when extended, now defines the `run!` method on the gem's
  root namespace.
- This is simply a delegation to Kangaru.application.run!, but allows
  calling the application from the target gem's namespace
  (eg SomeGem.run!)

lib/kangaru/initialiser.rb

@@ -4,6 +4,12 @@ def self.extended(namespace)
       root_file = caller[0].gsub(/:.*$/, "")
 
       Kangaru.application = Application.new(root_file:, namespace:).tap(&:setup)
+
+      namespace.module_eval do
+        def self.run!(argv)
+          Kangaru.application.run!(argv)
+        end
+      end
     end
   end
 end

spec/features/initialiser_spec.rb

@@ -36,6 +36,11 @@ module SomeGem
     it "does not set the Kangaru application reference" do
       expect { require_gem }.not_to change { Kangaru.application }.from(nil)
     end
+
+    it "does not define the run! method in the gem's root module" do
+      require_gem
+      expect(SomeGem).not_to respond_to(:run!)
+    end
   end
 
   context "when the target gem extends the initialiser" do
@@ -70,6 +75,11 @@ module SomeGem
         .to(a_kind_of(Kangaru::Application))
     end
 
+    it "defines the run! method in the gem's root module" do
+      require_gem
+      expect(SomeGem).to respond_to(:run!)
+    end
+
     describe "application reference" do
       subject(:application) { Kangaru.application }