For blessed gems, shim their commands so users know how to use them.

bin/jekyll

@@ -6,10 +6,6 @@ $LOAD_PATH.unshift File.join(File.dirname(__FILE__), *%w( .. lib ))
 require 'jekyll'
 require 'mercenary'
 
-Jekyll::External.require_if_present(
-  Jekyll::External.blessed_gems
-)
-
 Jekyll::PluginManager.require_from_bundler
 
 Jekyll::Deprecator.process(ARGV)
@@ -26,6 +22,16 @@ Mercenary.program(:jekyll) do |p|
   p.option 'layouts_dir', '--layouts DIR', String, 'Layouts directory (defaults to ./_layouts)'
   p.option 'profile', '--profile', 'Generate a Liquid rendering profile'
 
+  Jekyll::External.require_if_present(Jekyll::External.blessed_gems) do |g|
+    cmd = g.split('-').last
+    p.command(cmd.to_sym) do |c|
+      c.syntax cmd
+      c.action do
+        Jekyll.logger.abort_with "You must install the '#{g}' gem to use the 'jekyll #{cmd}' command."
+      end
+    end
+  end
+
   Jekyll::Command.subclasses.each { |c| c.init_with_program(p) }
 
   p.action do |args, _|

lib/jekyll/external.rb

@@ -17,12 +17,13 @@ module Jekyll
       #
       # names - a string gem name or array of gem names
       #
-      def require_if_present(names)
+      def require_if_present(names, &block)
         Array(names).each do |name|
           begin
             require name
           rescue LoadError
             Jekyll.logger.debug "Couldn't load #{name}. Skipping."
+            block.call(name) if block
             false
           end
         end