Improve & standardize help output

[rgoldberg]

Improve & standardize help output.

Specifically focus on what Apple IDs are used for each subcommand:

• macOS Apple ID, which is associated with the current macOS user in System Settings/System Preferences. It can be observed:
  • In Swift:

  (UserDefaults.standard.persistentDomain(forName: "MobileMeAccounts")?["Accounts"] as? [[String: Any]])?.first?["AccountID"]
  

  • From command line:

  defaults read MobileMeAccounts | plutil -extract "Accounts".0."AccountID" raw -o - -
  

• App Store Apple ID, which is when you login in the App Store app. It can be observed:
  • In Swift (in macOs <= 10.12.x):

  ISStoreAccount.primaryAccount.identifier
  

  • From command line: not sure

Currently, I think that the subcommands, when working, have the following interactions with the macOS & MAS Apple IDs:

• MAS
  • account
  • signin
  • signout
• MAS (requires app in macOS installed app list)
  • upgrade
• MAS (requires app not in macOS installed app list)
  • install
  • lucky
  • purchase
• macOS (requires app in macOS installed app list)
  • list
  • outdated
  • uninstall (currently used, but probably not strictly necessary)
• Neither
  • help
  • home
  • info
  • open
  • reset
  • search
  • vendor
  • version

Also, standardize various variable names to conform to standardized naming in help, documentation, etc.

Will do so after upgrading to Swift Argument Parser, so waiting for #542 to be merged.