TODO
  o Check on return value from #process block to see if all options was handled:
      case opt
        when '-v'; verbose = true # Return value 'true' is ok
        # Unhandled option means return value is nil
      end
  o Consolidate some of the 3 variations of #error and #fail
  o Add a option flag for solitary options (--help) 
  o Make a #to_yaml
  o Make an official dump method for debug
  o Make a note that all options are processed at once and not as-you-go
  o Test that arguments with spaces work
  o Long version usage strings
  o Doc: Example of processing of sub-commands and sub-sub-commands

  + More tests
  + More doc
  + Implement value-name-before-flags rule
  + Kill option array values
  + Kill key forms
  + Rename Option#opt to Option#name
  + Have all Ast objects to be on [key, name, value] form
  + Change #=>i, $=>f and introduce b (boolean)
  + Unshift program name to usage definition string before compiling
  + Rename to UsageCompiler and ArgvParser
  + Make usage-string handle commands
  + Change !cmd to cmd!
  + Clean-up terminology: Option-name is used for names with and without the prefixed dashes
  + Rename Option#has_argument? and #optional? to something else
  + Fix location reporting of compiler errors
  + Allow '--' in usage so that everything after can be used as USAGE in error messages
  + Handle pretty-printing of usage string in handling of ParserError
  + Compiler.new.compile(usage), Parser.new(compiled_program).parse(argv)
  + Check for duplicate option in the parser
  + Handle CompilerError
  + Use nil value as the name of the top 'command'
  + Refactor compilation to avoid having the Command objects throw CompilerErrors
  + Change to 'parser.parse' / 'parser.parse3'
  + Use first long option as symbolic key
  + Use full option names everywhere (eg. '--all' instead of 'all')

  - Revert change from '#' -> 'i'
  - Guard against reserved 'object_id' name in OpenStruct
  - Default value ('=' -> ':')
      Default values are better handled in the calling program

  ? More specialized exceptions: "MissingArgumentError" etc.

LATER
  o Allow '-a' and '--aa' in usage
  o Allow single-line comments
  o Allow multi-line comments
  o Regex as option value spec
  o "FILE", "DIR", "NEWFILE", "NEWDIR" as keyword in option value spec
      RFILE, RDIR
      WFILE, WDIR
      EFILE, EDIR
  o Octal and hexadecimal integers
  o Escape of separator in lists
  o Handle output of subcommand usage like "cmd1 cmd1.cmd2 cmd2"
  o Command-specific arguments: clone! o,opt ++ ARG1 ARG2...
  o Hostname and email as basic types

ON TO_H BRANCH
  ShellOpts.process(usage, argv) { |opt,val| ... } => args
  ShellOpts.process(usage, argv) { |key,opt,val| ... } => args

  opts = ShellOpts.new(usage, argv, defaults = {})
  opts = ShellOpts.new(usage, argv, defaults = OpenStruct.new)

  opts.args
  opts.to_a
  opts.to_h
  opts.to_openstruct

  opts.each { |opt,val| ... }
  opts.each { |key,opt,val| ... }

LONG FORMAT

  PROGRAM = File.basename(ARGV.first)
  USAGE = "-a -f FILE -lvh FILE..."
  DESCR = %(
    Short description

    Longer description
  )
  OPTIONS = %(
    -a,--all
      Process all files

    -f, --file=FILE
      Process file

    !command
      This is a command

      --this-is-a-command-option
        Options for commands are nested

    ...
  )