args: ability to specify 'help' for commands

[DartBot]

This issue was originally filed by @seaneagan

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If we can add commands to a parser, we ought to be able to add 'help' text for those commands (as with options and flags), and get it back out in a nice format.

    parser.addCommand(... help: ...);

    print(parser.commandUsage);

Which might output something like:

start Starts the service
stop Stops the service
restart Restarts the service

The help text could be accessible via a field in the command's ArgParser:

    parser.commands['foo'].help

and this field could be reused to store a description of the overall script:

    parser.help

(or possibly s/help/description)

[iposva-google]

Added Area-Pkg, Library-Args, Triaged labels.

[anders-sandholm]

Removed Library-Args label.
Added Pkg-Args label.

[DartBot]

This issue has been moved to dart-lang/core#122.