Add support for customizing default value string displayed in Help (#87)

[griffin-stewie]

• I've completed this task

This PR fixes #87.

Plan

Simply we can add defaultValueDescription in ExpressibleByArgument protocol

/// A type that can be expressed as a command-line argument.
public protocol ExpressibleByArgument {
  /// Creates a new instance of this type from a command-line-specified
  /// argument.
  init?(argument: String)

  /// Add this ↓
  var defaultValueDescription: String { get }
}

Source Impact

Default behavior is given by default implementation declared in ExpressibleByArgument as Protocol Extensions. Only users who want to customize need to override defaultValueDescription.

Related Forum Thread

How can I change the default value string displayed in Help? - Related Projects / ArgumentParser - Swift Forums

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

One small nit, then this looks good to go!

[natecook1000]

Nit: Can you indent with 2 spaces instead of 4?

[griffin-stewie]

Oops, I fixed it now.

[natecook1000]

🚢