Add 'jekyll help' command.

Ref: #2695

lib/jekyll/commands/help.rb

@@ -0,0 +1,30 @@
+module Jekyll
+  module Commands
+    class Help < Command
+      class << self
+
+        def init_with_program(prog)
+          prog.command(:help) do |c|
+            c.syntax 'help <command>'
+            c.description 'Show the help for'
+
+            c.action do |args, _|
+              if args.empty?
+                puts prog
+              else
+                puts prog.commands[args.first.to_sym]
+              end
+            end
+          end
+        end
+
+        def usage_message(prog, cmd)
+          Jekyll.logger.error "Error:", "No command specified."
+          Jekyll.logger.warn  "Usage:", cmd.syntax
+          Jekyll.logger.info  "Valid commands:", prog.commands.keys.join(", ")
+        end
+
+      end
+    end
+  end
+end