Add package_name_format field to recipe

Setting the field to a format string value changes the filename of the
created package. If unset, FPM chooses its default format.

See the following code section for available format strings:
https://github.com/jordansissel/fpm/blob/c40f6818f88a6dd3df7bc19b6007b03a716949f8/lib/fpm/package.rb#L355-L365

Fixes #210

lib/fpm/cookery/packager.rb

@@ -213,18 +213,19 @@ def build_package(recipe, config)
 
           output = input.convert(output_class)
 
+          output_filename = output.to_s(recipe.package_name_format)
           recipe.run_lifecycle_hook(:before_package_create, output)
           begin
-            output.output(output.to_s)
+            output.output(output_filename)
             recipe.run_lifecycle_hook(:after_package_create, output)
           rescue FPM::Package::FileAlreadyExists
-            Log.info "Removing existing package file: #{output.to_s}"
-            FileUtils.rm_f(output.to_s)
+            Log.info "Removing existing package file: #{output_filename}"
+            FileUtils.rm_f(output_filename)
             retry
           ensure
             input.cleanup if input
             output.cleanup if output
-            Log.info "Created package: #{File.join(Dir.pwd, output.to_s)}"
+            Log.info "Created package: #{File.join(Dir.pwd, output_filename)}"
           end
         end
       end

lib/fpm/cookery/recipe.rb

@@ -35,7 +35,7 @@ class BaseRecipe
               :pre_install, :post_install, :pre_uninstall, :post_uninstall,
               :license, :omnibus_package, :omnibus_dir, :chain_package,
               :default_prefix, :docker, :docker_image, :dockerfile,
-              :epoch
+              :package_name_format, :epoch
 
       attr_rw_list  :build_depends, :config_files, :conflicts, :depends,
                     :exclude, :patches, :provides, :replaces, :omnibus_recipes,