refactor(video2x): split the CLI into multiple files; improve CLI args validation (#1247)

Signed-off-by: k4yt3x <i@k4yt3x.com>

include/libvideo2x/processor_factory.h

@@ -1,5 +1,4 @@
-#ifndef PROCESSOR_FACTORY_H
-#define PROCESSOR_FACTORY_H
+#pragma once
 
 #include <functional>
 #include <memory>
@@ -32,5 +31,3 @@ class ProcessorFactory {
     // Static initializer for default processors
     static void init_default_processors(ProcessorFactory &factory);
 };
-
-#endif  // PROCESSOR_FACTORY_H