all commands: prettify/customize Cobra usage

Reference: https://github.com/spf13/cobra#defining-your-own-usage

Defaults can be found here: https://github.com/spf13/cobra/blob/master/command.go

Start with Cobra's “Defining your own usage” documentation and the defaults in command.go, then inspect how this CLI constructs its commands. Define the desired usage format for all commands and verify that each command presents the customized output consistently.

索引モデルが issue の本文から書いたものです。