[ruby/irb] Allow defining custom commands in IRB

(https://github.com/ruby/irb/pull/886)

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

Example usage:

```ruby
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)
```

https://github.com/ruby/irb/commit/888643467c

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