Require gems in :jekyll_plugins group in a Gemfile in unsafe mode.

Replaces #1658. /cc @imathis

lib/jekyll/plugin_manager.rb

@@ -17,6 +17,7 @@ module Jekyll
     def conscientious_require
       require_plugin_files
       require_gems
+      require_from_bundler
     end
 
     # Require each of the gem plugins specified.
@@ -30,6 +31,15 @@ module Jekyll
       end
     end
 
+    def require_from_bundler
+      unless site.safe
+        require "bundler/setup"
+        Bundler.require(:jekyll_plugins)
+      end
+    rescue LoadError
+      false
+    end
+
     # Check whether a gem plugin is allowed to be used during this build.
     #
     # gem_name - the name of the gem