Introduces defaultToHelp command configuration

[erica]

Adds configuration option that allows a command to present help when
no other input is provided by the user at the command-line call.

Description

This change introduces a hook so users can configure the command to
respond with a help message on the absence of arguments.

Detailed Design

// CommandConfiguration:
// ...
  /// A Boolean value indicating whether this command should display help
  /// when executed without further user input.
  public var defaultToHelp: Bool
// ...

Documentation Plan

Updated code doc in CommandConfiguration.swift

Test Plan

Test with updated math.swift. Instead of returning 0 without arguments, it should return the help message.

Source Impact

This is purely additive and optional, defaulting to false, the existing behavior.

Checklist

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

I have not updated the documentation, but will amend this PR if the idea has a chance for moving forward.

[natecook1000]

Hi @erica — thanks for the PR! We’re trying to constrain the configuration options to only things that aren’t really possible to handle from within client code, so I don’t think this is an option we want to add right now.

To get this functionality yourself, you can throw CleanExit.helpRequest() from your run() or validate methods.

There’s a very-closely related issue covered in #134 if you’d like to tackle that. We also will need an API to directly generate the help text from a command.

[erica]

throw CleanExit.helpRequest() worked beautifully for my immediate needs.

[natecook1000]

I think this is addressed by #140 and #142 — feel free to re-open if you want to discuss any more!