Allow the user to set collections_dir to put all collections under one subdirectory

docs/_docs/configuration.md

@@ -602,12 +602,13 @@ file or on the command-line.
 
 ```yaml
 # Where things are
-source:       .
-destination:  ./_site
-plugins_dir:  _plugins
-layouts_dir:  _layouts
-data_dir:     _data
-includes_dir: _includes
+source:          .
+destination:     ./_site
+collections_dir: .
+plugins_dir:     _plugins
+layouts_dir:     _layouts
+data_dir:        _data
+includes_dir:    _includes
 collections:
   posts:
     output:   true

lib/jekyll/collection.rb

@@ -100,15 +100,15 @@ def filtered_entries
     # Returns a String containing the directory name where the collection
     #   is stored on the filesystem.
     def relative_directory
-      @relative_directory ||= "_#{label}"
+      @relative_directory ||= Pathname.new(directory).relative_path_from(Pathname.new(site.source)).to_s
     end
 
     # The full path to the directory containing the collection.
     #
     # Returns a String containing th directory name where the collection
     #   is stored on the filesystem.
     def directory
-      @directory ||= site.in_source_dir(relative_directory)
+      @directory ||= site.in_source_dir(File.join(site.config["collections_dir"], "_#{label}"))
     end
 
     # The full path to the directory containing the collection, with

lib/jekyll/configuration.rb

@@ -9,6 +9,7 @@ class Configuration < Hash
       # Where things are
       "source"              => Dir.pwd,
       "destination"         => File.join(Dir.pwd, "_site"),
+      "collections_dir"     => "",
       "plugins_dir"         => "_plugins",
       "layouts_dir"         => "_layouts",
       "data_dir"            => "_data",