ArgumentParser 0.3.1

Fixes

• An option or flag can now declare a name with both single- and double-
  dash prefixes, such as -my-flag and --my-flag. Specify both names in the
  name parameter when declaring your property:

  @Flag(name: [.long, .customLong("my-flag", withSingleDash: true)])
  var myFlag = false

• Parsing performance improvements.