Allow plugins to modify the obsolete files. (#6502)

Merge pull request 6502

lib/jekyll/cleaner.rb

@@ -24,7 +24,9 @@ module Jekyll
     #
     # Returns an Array of the file and directory paths
     def obsolete_files
-      (existing_files - new_files - new_dirs + replaced_files).to_a
+      out = (existing_files - new_files - new_dirs + replaced_files).to_a
+      Jekyll::Hooks.trigger :clean, :on_obsolete, out
+      out
     end
 
     # Private: The metadata file storing dependency tree and build history

lib/jekyll/hooks.rb

@@ -39,6 +39,9 @@ module Jekyll
         :post_render => [],
         :post_write  => [],
       },
+      :clean     => {
+        :on_obsolete => [],
+      },
     }
 
     # map of all hooks and their priorities