Support command registration

[st0012]

Overview

This is a feature that has been requested for a long time. It is now possible to define custom commands in IRB.

Example usage:

require "irb/command"

class HelloCommand < IRB::Command::Base
  description "Prints hello world"
  category "My commands"
  help_message "It doesn't do more than printing hello world."

  def execute
    puts "Hello world"
  end
end

IRB::Command.register(:hello, HelloCommand)

And the command will appear in the help message:

Help
  help           List all available commands. Use `help <command>` to get information about a specific command.

My commands
  hello          Prints hello world

IRB
  ....

Details

• All the default command definition stuff is now moved to lib/irb/default_commands.rb so requiring irb/command is very lightweight.
• 3 Command APIs are introduced
  • Command.register(command_name, command_class) is the official API to register a command. I decided not to accept alias for now as the current aliasing mechanism is messy.
  • Command._register_with_aliases(name, command_class, *aliases) is an internal API for IRB to register its existing commands and their aliases.
  • Command.commands to get all the registered commands
• Many command handling methods are simplified with the change as well.

I think this is should probably be introduced before #624 because:

• It already simplifies IRB's command registration and will make future refactor easier
• I still haven't figured out how to better display helper methods in the help message
• The community is already familiar with the concept of commands, so I imagine this will be easy to adopt. And with custom commands, it'll help both us and the community understand the role of helper methods.

Closes #513

[tompng]

I think this is basically OK 👍
One concern is that once we release this public customizing API, fixing #592 might be harder than before (to keep public API backward compatibility).

What do you think? I think I'm ready with #824 that fixes #592.

I don't want to make transform_args a public API. (We can restrict only for default commands)
If we use the method name execute(*ruby_args, **ruby_kwargs), execution method for raw string that previously used transform_args should be a different name like execute_raw(string_arg). I think this method should be named execute.

[st0012]

@tompng I've rebased the PR and made a few more improvements. Can you give it another look?

[tompng]

👍