add `bundle install` to `jekyll new`

- automatically run `bundle install` from within the newly generated blog
   directory by default.
 - add a new switch to skip this default behaviour.

lib/jekyll/commands/new.rb

@@ -11,6 +11,7 @@ module Jekyll
 
             c.option "force", "--force", "Force creation even if PATH already exists"
             c.option "blank", "--blank", "Creates scaffolding but with empty files"
+            c.option "skip-bundle", "--skip-bundle", "Skip 'bundle install'"
 
             c.action do |args, options|
               Jekyll::Commands::New.process(args, options)
@@ -34,7 +35,7 @@ module Jekyll
             create_site new_blog_path
           end
 
-          Jekyll.logger.info "New jekyll site installed in #{new_blog_path.cyan}."
+          after_install(new_blog_path, options)
         end
 
         def create_blank_site(path)
@@ -114,6 +115,27 @@ RUBY
         def scaffold_path
           "_posts/0000-00-00-welcome-to-jekyll.markdown.erb"
         end
+
+        # After a new blog has been created, print a success notification and
+        # then automatically execute bundle install from within the new blog dir
+        # unless the user opts to generate a blank blog or skip 'bundle install'.
+
+        def after_install(path, options = {})
+          Jekyll.logger.info "New jekyll site installed in #{path.cyan}."
+          Jekyll.logger.info "Bundle install skipped." if options["skip-bundle"]
+
+          unless options["blank"] || options["skip-bundle"]
+            bundle_install path
+          end
+        end
+
+        def bundle_install(path)
+          Jekyll::External.require_with_graceful_fail "bundler"
+          Jekyll.logger.info "Running bundle install in #{path.cyan}..."
+          Dir.chdir(path) do
+            system("bundle", "install")
+          end
+        end
       end
     end
   end