Merge pull request #7607 from reitermarkus/cask-help-usage

Implement `brew cask help <command>`.

Library/Homebrew/cask/cmd.rb

@@ -145,7 +145,7 @@ module Cask
       command, args = detect_internal_command(*args) || detect_external_command(*args) || [NullCommand.new, args]
 
       if help?
-        puts command.help
+        Help.new(command.command_name).run
       else
         command.run(*args)
       end

Library/Homebrew/cask/cmd/help.rb

@@ -3,17 +3,26 @@
 module Cask
   class Cmd
     class Help < AbstractCommand
-      def initialize(*)
-        super
-        return if args.empty?
-
-        raise ArgumentError, "#{self.class.command_name} does not take arguments."
-      end
-
       def run
-        puts self.class.purpose
-        puts
-        puts self.class.usage
+        if args.empty?
+          puts self.class.purpose
+          puts
+          puts self.class.usage
+        elsif args.count == 1
+          command_name = args.first
+
+          unless command = self.class.commands[command_name]
+            raise "No help information found for command '#{command_name}'."
+          end
+
+          if command.respond_to?(:usage)
+            puts command.usage
+          else
+            puts command.help
+          end
+        else
+          raise ArgumentError, "#{self.class.command_name} only takes up to one argument."
+        end
       end
 
       def self.purpose
@@ -23,6 +32,10 @@ module Cask
         EOS
       end
 
+      def self.commands
+        Cmd.command_classes.select(&:visible?).map { |klass| [klass.command_name, klass] }.to_h
+      end
+
       def self.usage
         max_command_len = Cmd.commands.map(&:length).max