Add mailbox generator feature

Author: vivekmiyani

features/generator_specs/mailbox_specs.feature

@@ -0,0 +1,25 @@
+Feature: Mailbox generator spec
+
+    Scenario: Mailbox generator
+        When I run `bundle exec rails generate mailbox forwards`
+        Then the features should pass
+        Then the output should contain:
+          """
+                create  app/mailboxes/forwards_mailbox.rb
+                invoke  rspec
+                create    spec/mailboxes/forwards_mailbox_spec.rb
+          """
+
+    Scenario: Mailbox generator with customized `default-path`
+        Given a file named ".rspec" with:
+          """
+          --default-path behaviour
+          """
+        And I run `bundle exec rails generate mailbox forwards`
+        Then the features should pass
+        Then the output should contain:
+          """
+                create  app/mailboxes/forwards_mailbox.rb
+                invoke  rspec
+                create    behaviour/mailboxes/forwards_mailbox_spec.rb
+          """