Display a description and a quick usage message

[PoloLacoste]

Would it be usefull to add a quick usage message and a description at the top of the usage message.

For instance, the example in 'example/test_runner.dart' would look like this :

usage: [-c] [-r] [-a] [-s] [-m] [--[no-]checked] [--[no-]host-checked] [-t] [-j] [--shards] [--shard] [--[no-]valgrind] [-p] [--[no-]report] [-v] [--[no-]list] [--[no-]time] [-b] [--[no-]keep-generated-tests] [--special-command] [--dart] [--drt] [--dartium] [--mandatory]

A simple description

===== Platform
-c, --compiler                     Specify any compilation step (if needed).
[...]

[munificent]

I think most users simply print the description themselves before printing the usage information:

A simple description

usage: [-c] [-r] [-a] [-s] [-m] [--[no-]checked] [--[no-]host-checked] [-t] [-j] [--shards] [--shard] [--[no-]valgrind] [-p] [--[no-]report] [-v] [--[no-]list] [--[no-]time] [-b] [--[no-]keep-generated-tests] [--special-command] [--dart] [--drt] [--dartium] [--mandatory]

===== Platform
-c, --compiler                     Specify any compilation step (if needed).
[...]