JM REMEUR

Results 2 issues of JM REMEUR

As mentioned in #194 there is a bug when you use the mandatory flag in an option with an help flag (to display the usage message). Example (from [here](https://stackoverflow.com/questions/67384823/how-to-use-mandatory-field-in-dart-args-package)): ```dart...

cla: yes

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...