* AddHelpCategory() was removed in favor of an exported
   HelpCategories global.
 * AddCommand() now accepts a CmdInfo struct as its sole
   argument.

 * Removed unused struct members such as Aliases and Hidden.
 * Replace the Extra function with PassAfterNonOption, as this
   is the only internal flags.Command option that justified its
   need.
 * Rename OptDesc/ArgDesc for clarity.
 * Document the public API.

 * CmdInfo.{ShortHelp,LongHelp} -> CmdInfo.{Summary,Description}
 * CmdInfo.PositionalArgs -> CmdInfo.ArgumentsHelp
 * PositionalArg.{Name,ShortHelp} ->
       ArgumentHelp.{Placeholder,Help}

 * CmdInfo.ArgsHelp now contains the help text for short, long options
   and positional arguments, using a new convention.
 * func init() on all commands is now moved to the top of the source
   file, and help strings no longer are globals, for better readability
   and for immediate information about the command when reading from
   the top.
 * Remove debug command (which duplicated Parser() functionality)
 * Refactor Parser()
 * Still needs fix: broken workaround that has been removed, which
   previously overrode the usage string from the parser so that
   help manuals don't introduce the [<command>-OPTIONS] string on
   the usage line. This is a bug in both the CLI help and manpage
   generation code in go-flags, and will be tackled in a future
   PR in canonical/go-flags. See canonical/go-flags#4

 * Strip some dead code
 * Minor corrections to keep staticcheck happy.

The following flags will pass: `-a`, `--long-flag`, `--b`, `--this-is-a-flag`;
the following flags won't: `---`, `---c`, `--quasi--valid`