Allow defining a help string for an option's parameter value.

pkg/args/README.md

 Parses raw command-line arguments into a set of options and values.
 
 This library supports [GNU][] and [POSIX][] style options, and it works
 in both server-side and client-side apps.
 
 ## Defining options
 
 First create an [ArgParser][]:
 
     var parser = new ArgParser();
@@ skipping 191 matching lines @@
 You can automatically generate nice help text, suitable for use as the output of
 `--help`. To display good usage information, you should provide some help text
 when you create your options.
 
 To define help text for an entire option, use the `help:` parameter:
 
     parser.addOption('mode', help: 'The compiler configuration',
         allowed: ['debug', 'release']);
     parser.addFlag('verbose', help: 'Show additional diagnostic info');
 
+For non-flag options, you can also provide a help string for the parameter:

[nweiz, 2014/07/02 19:41:32]

"help string" -> "name"

[Bob Nystrom, 2014/07/07 20:37:34]

I think I like "help string" here. I want people to understand the name doesn't
affect how they access the arg programmatically.

[nweiz, 2014/07/07 20:52:00]

Again, I think "help" is strongly tied to prose descriptions.

+
+    parser.addOption('out', help: 'The output path', helpValue: 'path',
+        allowed: ['debug', 'release']);
+
 For non-flag options, you can also provide detailed help for each expected value
 by using the `allowedHelp:` parameter:
 
     parser.addOption('arch', help: 'The architecture to compile for',
         allowedHelp: {
           'ia32': 'Intel x86',
           'arm': 'ARM Holding 32-bit chip'
         });
 
 To display the help, use the [getUsage()][getUsage] method:
 
     print(parser.getUsage());
 
 The resulting string looks something like this:
 
     --mode            The compiler configuration
                       [debug, release]
 
+    --out=<path>      The output path
     --[no-]verbose    Show additional diagnostic info
     --arch            The architecture to compile for
-
           [arm]       ARM Holding 32-bit chip
           [ia32]      Intel x86
 
-To assist the formatting of the usage help, single-line help text is followed by
-a single new line. Options with multi-line help text are followed by two new
-lines. This provides spatial diversity between options.

[nweiz, 2014/07/02 19:41:32]

This change should probably also be mentioned in the changelog.

[Bob Nystrom, 2014/07/07 20:37:34]

There's no behavioral change here. I just took the bit out of the README because
I never really liked the wording of that paragraph. :(

-
 [posix]: http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap12.html#tag_12_02
 [gnu]: http://www.gnu.org/prep/standards/standards.html#Command_002dLine-Interfaces
 [ArgParser]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgParser
 [ArgResults]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgResults
 [addOption]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgParser#id_addOption
 [addFlag]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgParser#id_addFlag
 [parse]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgParser#id_parse
 [rest]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgResults#id_rest
 [addCommand]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgParser#id_addCommand
 [getUsage]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgParser#id_getUsage