[update] implement '-U/--update'

to auto-update executable files in-place

gallery_dl/__init__.py

@@ -183,7 +183,13 @@ def main():
             else:
                 extractor._module_iter = iter(modules[0])
 
-        if args.list_modules:
+        if args.update:
+            from . import update
+            extr = update.UpdateExtractor.from_url("update:" + args.update)
+            ujob = update.UpdateJob(extr)
+            return ujob.run()
+
+        elif args.list_modules:
             extractor.modules.append("")
             sys.stdout.write("\n".join(extractor.modules))