(https://github.com/ruby/irb/pull/877)
* Make help command display help for individual commands
Usage: `help [command]`
If the command is not specified, it will display a list of all available commands.
If the command is specified, it will display the banner OR description of the command.
If the command is not found, it will display a message saying that the command is not found.
* Rename test/irb/cmd to test/irb/command
* Add banner to edit and ls commands
* Promote help command in the help message
1. Make `show_cmds` an alias of `help` so it's not displayed in the help message
2. Update description of the help command to reflect `help <command>` syntax
* Rename banner to help_message
https://github.com/ruby/irb/commit/43a2c99f3f