Add @Flag initializer for Optional<Bool> (#54)

* Add @Flag initializer for Optional<Bool>

Per [discussion][], add an initializer for `Flag<Optional<Bool>>` that
allows the default value to be `nil`, enabling capacity to distinguish
the lack of a supplied flag vs the default value from user.

[discussion]: https://forums.swift.org/t/flag-default-value-where-did-it-come-from/34204/10

* fixup! Add @Flag initializer for Optional<Bool>

Author: dduan

Sources/ArgumentParser/Parsable Properties/Flag.swift

@@ -112,6 +112,35 @@ public enum FlagExclusivity {
   case chooseLast
 }
 
+extension Flag where Value == Optional<Bool> {
+  /// Creates a Boolean property that reads its value from the presence of
+  /// one or more inverted flags.
+  ///
+  /// Use this initializer to create an optional Boolean flag with an on/off
+  /// pair. With the following declaration, for example, the user can specify
+  /// either `--use-https` or `--no-use-https` to set the `useHTTPS` flag to
+  /// `true` or `false`, respectively. If neither is specified, the resulting
+  /// flag value would be `nil`.
+  ///
+  ///     @Flag(inversion: .prefixedNo)
+  ///     var useHTTPS: Bool?
+  ///
+  /// - Parameters:
+  ///   - name: A specification for what names are allowed for this flag.
+  ///   - inversion: The method for converting this flags name into an on/off
+  ///     pair.
+  ///   - help: Information about how to use this flag.
+  public init(
+    name: NameSpecification = .long,
+    inversion: FlagInversion,
+    help: ArgumentHelp? = nil
+  ) {
+    self.init(_parsedValue: .init { key in
+      .flag(key: key, name: name, default: nil, inversion: inversion, help: help)
+    })
+  }
+}
+
 extension Flag where Value == Bool {
   /// Creates a Boolean property that reads its value from the presence of a
   /// flag.

Tests/EndToEndTests/FlagsEndToEndTests.swift

@@ -24,25 +24,37 @@ fileprivate struct Bar: ParsableArguments {
 
   @Flag(inversion: .prefixedNo)
   var extattr: Bool
+
+  @Flag(inversion: .prefixedNo)
+  var extattr2: Bool?
 }
 
 extension FlagsEndToEndTests {
   func testParsing_defaultValue() throws {
     AssertParse(Bar.self, []) { options in
       XCTAssertEqual(options.verbose, false)
       XCTAssertEqual(options.extattr, false)
+      XCTAssertEqual(options.extattr2, nil)
     }
   }
 
   func testParsing_settingValue() throws {
     AssertParse(Bar.self, ["--verbose"]) { options in
       XCTAssertEqual(options.verbose, true)
       XCTAssertEqual(options.extattr, false)
+      XCTAssertEqual(options.extattr2, nil)
     }
 
     AssertParse(Bar.self, ["--extattr"]) { options in
       XCTAssertEqual(options.verbose, false)
       XCTAssertEqual(options.extattr, true)
+      XCTAssertEqual(options.extattr2, nil)
+    }
+
+    AssertParse(Bar.self, ["--extattr2"]) { options in
+      XCTAssertEqual(options.verbose, false)
+      XCTAssertEqual(options.extattr, false)
+      XCTAssertEqual(options.extattr2, .some(true))
     }
   }