added --installed-plugins switch to pry binary to list installed plugins with description

bin/pry

@@ -39,6 +39,8 @@ See: `https://github.com/banister` for more information.
     Pry.config.plugins.enabled = false
   end
 
+  on "installed-plugins", "List installed plugins."
+
   on "simple-prompt", "Enable simple prompt mode" do
     Pry.prompt = Pry::SIMPLE_PROMPT
   end
@@ -63,6 +65,15 @@ See: `https://github.com/banister` for more information.
     )
 end
 
+if opts["installed-plugins"]
+  puts "Installed Plugins:"
+  puts "--"
+  Pry.locate_plugins.each do |plugin|
+    puts "#{plugin.name}".ljust(18) + plugin.spec.summary
+  end
+  exit
+end
+
 # invoked via cli
 Pry.cli = true