Add argument groups

[seaneagan]

Grouping arguments would allow for over-arching documentation for specific groups of arguments.

Might be able to use a similar interface as #33 for this. Possibly only annotate first arg of group, and assume all args until next group annotation are in that group:

script({
  int timeout,
  // ... other options that don't belong to a specific group.
  @HelpBlock('Formatting options', 'Options to tailor the output')
  bool indent,
  String separator: ' ',
  //...
}) {

}

Positional arguments should just be in their own single group, since there should never be more than 2 or 3 of them.

Possibly related dart feature request, which might be able to provide a less command-line specific interface for this: http://dartbug.com/2783

Also, flag groups could be made mutually exclusive where only one option from the group can be specified, otherwise a helpful error message is displayed. For example, there might be multiple verbosity options like --quiet, --verbose, --trace etc, see #4. These options could potentially be modeled by an enum (when dart supports those), and a single enum value could be passed to the script method,

class Verbosity {

  final Level;

  const Verbosity._(this.level);

  static const Verbosity 
      QUIET = const Verbosity._(Level.OFF),
      VERBOSE = const Verbosity._(Level.FINE),
       YELL = const Verbosity._(Level.ALL);

  static const values =  const [QUIET, VERBOSE, OFF];
}

Still need to think about the interface to declare that a script takes one of these mutually exclusive flag groups. It might involve the allowed attribute, and be related to #13.

Note: Verbosity option groups may not actually be passed as a direct value, see #4.