Support positional arguments

[rvesse]

Currently @Arguments is a general annotation that can only be applied to a single field and collects all non-option values into that field. There are various use cases where it might be nicer to support true positional arguments i.e. a new @PositionalArgument annotation such that arguments could be captured on separate fields and provide more detailed help specific to each argument

e.g.

@Command(name = "pos-args")
public class PositionalArgumentExample {

  @PositionalArgument(title = "Source", description = "The source file", position = { 0 })
  private String source;

  @PositionalArgument(title = "Target", description = "The target file", position = { 1 })
  private String target;

  @Arguments(title = "ExtraArg", description = "Extra arguments")
  private List<String> extraArgs = new ArrayList();
}

Which would be used as follows:

> pos-args source.txt target.txt foo bar

Where source.txt would be set to the source field, target.txt set to the target field and the additional arguments passed to the extraArgs field

[rvesse]

Concrete tasks:

• New @PositionalArgument annotation
• New PositionalArgumentMetadata to hold the metadata for a positional argument
• Extend CommandMetadata with positional arguments support
• Extend AbstractCommandParser with positional arguments support
• Help generations updated to generate help for new positional arguments ArgumentMetadata
  • CLI
  • HTML
  • Markdown
  • Man
  • Bash
• ArgumentsRestriction extended to work with positional arguments as well
  • Implementations updated appropriately
  • Unit test coverage for the restrictions
• Migration Guide updated for behavioural changes around positional arguments