ArgumentParser 0.1.0

Additions

• Error messages and help screens now include information about how to request
  more help.
• CMake builds now support installation.

Changes

• The static func main() method on ParsableCommand no longer returns
  Never. This allows ParsableCommand types to be designated as the entry
  point for a Swift executable by using the @main attribute.

  Migration: For most uses, this change is source compatible. If you have
  used main() where a () -> Never function is explicitly required, you'll
  need to change your usage or capture the method in another function.

• Optional no longer conforms to ExpressibleByArgument, to avoid some
  property declarations that don't make sense.

  Migration: This is source-compatible for all property declarations, with
  deprecations for optional properties that define an explicit default. If
  you're using optional values where an ExpressibleByArgument type is
  expected, such as a generic function, you will need to change your usage
  or provide an explicit override.

• ParsableCommand's run() method requirement is now a mutating method,
  allowing mutations to a command's properties, such as sorting an array of
  arguments, without additional copying.

  Migration: No changes are required for commands that are executed through
  the main() method. If you manually parse a command and then call its
  run() method, you may need to change the command from a constant to a
  variable.

Removals

• The @Flag initializers that were deprecated in version 0.0.6 are now
  marked as unavailable.

Fixes

• @Option properties of an optional type that use a transform closure now
  correctly indicate their optionality in the usage string.
• Correct wrapping and indentation are maintained for abstracts and discussions
  with short lines.
• Empty abstracts no longer add extra blank lines to the help screen.
• Help requests are still honored even when a parsed command fails validation.
• The -- terminator isn't consumed when parsing a command, so that it can be
  parsed as a value when a subcommand includes an .unconditionalRemaining
  argument array.
• CMake builds work correctly again.