Add new C++ Command framework (#1785)

This is the C++ version of #1682.

The old command framework is still available, but will be deprecated.

Due to name conflicts, the new framework is in the frc2 namespace.
Eventually (after the old command framework is removed in a future year)
it will be moved into the main frc namespace.

wpilibc/src/main/native/include/frc2/command/StartEndCommand.h

@@ -0,0 +1,46 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2019 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include "CommandBase.h"
+#include "CommandHelper.h"
+
+namespace frc2 {
+/**
+ * A command that runs a given runnable when it is initalized, and another
+ * runnable when it ends. Useful for running and then stopping a motor, or
+ * extending and then retracting a solenoid. Has no end condition as-is; either
+ * subclass it or use Command.WithTimeout() or Command.InterruptOn() to give it
+ * one.
+ */
+class StartEndCommand : public CommandHelper<CommandBase, StartEndCommand> {
+ public:
+  /**
+   * Creates a new StartEndCommand.  Will run the given runnables when the
+   * command starts and when it ends.
+   *
+   * @param onInit       the Runnable to run on command init
+   * @param onEnd        the Runnable to run on command end
+   * @param requirements the subsystems required by this command
+   */
+  StartEndCommand(std::function<void()> onInit, std::function<void()> onEnd,
+                  std::initializer_list<Subsystem*> requirements);
+
+  StartEndCommand(StartEndCommand&& other) = default;
+
+  StartEndCommand(const StartEndCommand& other);
+
+  void Initialize() override;
+
+  void End(bool interrupted) override;
+
+ protected:
+  std::function<void()> m_onInit;
+  std::function<void()> m_onEnd;
+};
+}  // namespace frc2