Add mailer generator feature

Author: vivekmiyani

features/generator_specs/mailer_specs.feature

@@ -0,0 +1,43 @@
+Feature: Mailer generator spec
+
+    Scenario: Mailer generator
+        When I run `bundle exec rails generate mailer posts index show`
+        Then the features should pass
+        Then the output should contain:
+          """
+                create  app/mailers/posts_mailer.rb
+                invoke  erb
+                create    app/views/posts_mailer
+                create    app/views/posts_mailer/index.text.erb
+                create    app/views/posts_mailer/index.html.erb
+                create    app/views/posts_mailer/show.text.erb
+                create    app/views/posts_mailer/show.html.erb
+                invoke  rspec
+                create    spec/mailers/posts_spec.rb
+                create    spec/fixtures/posts/index
+                create    spec/fixtures/posts/show
+                create    spec/mailers/previews/posts_preview.rb
+          """
+
+    Scenario: Mailer generator with customized `default-path`
+        Given a file named ".rspec" with:
+          """
+          --default-path behaviour
+          """
+        And I run `bundle exec rails generate mailer posts index show`
+        Then the features should pass
+        Then the output should contain:
+          """
+                create  app/mailers/posts_mailer.rb
+                invoke  erb
+                create    app/views/posts_mailer
+                create    app/views/posts_mailer/index.text.erb
+                create    app/views/posts_mailer/index.html.erb
+                create    app/views/posts_mailer/show.text.erb
+                create    app/views/posts_mailer/show.html.erb
+                invoke  rspec
+                create    behaviour/mailers/posts_spec.rb
+                create    behaviour/fixtures/posts/index
+                create    behaviour/fixtures/posts/show
+                create    behaviour/mailers/previews/posts_preview.rb
+          """