Attribute parsing: 2.1.0-alpha

https://www.nuget.org/packages/McMaster.Extensions.CommandLineUtils/2.1.0-alpha

2.1.0:
New features:

• Attributes. Simplify command line argument definitions by adding attributes to a class that represents options and arguments.
  • Options defined as [Option] or [Argument], [Subcommand].
  • Command parsing options can be defined with [Command] and [Subcmomand].
  • Special options include [HelpOption] and [VersionOption].
• Async from end to end. Using C# 7.1 and attribute binding, your console app can be async from top to bottom.

New API

• Added OptionAttribute, ArgumentAttribute, CommandAttribute, SubcommandAttribute, HelpOptionAttribute, and VersionOptionAttribute.
• CommandLineApplication.Execute<TApp>() - executes an app where TApp uses attributes to define its options
• CommandLineApplication.ExecuteAsync<TApp>() - sample thing, but async.
• CommandLineApplication.StopParsingHelpOption and StopParsingVerboseOption. When the help and verbose options are matched
  against a command-line flag, the parsing will stop by default. You can turn this off by setting these options if you
  want OnExecute to be invoked no matter what.
• CommandLineApplication.HandleResponseFiles - the parser can treat arguments that begin with '@' as response files.
  Response files contain arguments that will be treated as if they were passed on command line.

Minor bug fixes:

• Add return types to .VerboseOption() and ensure .HasValue() is true when HelpOption or VerboseOption are matched
• Fix a NullReferenceException in some edge cases when parsing args