Add steal to lockable

src/Lockable.h

@@ -12,6 +12,9 @@ namespace Feather
         auto borrow()       { return std::pair<      T&, std::lock_guard<std::mutex>>{object, mutex}; }
         auto borrow() const { return std::pair<const T&, std::lock_guard<std::mutex>>{object, mutex}; }
 
+              T& steal()       { return object; }
+        const T& steal() const { return object; }
+
     private:
         T                  object;
         mutable std::mutex mutex;