Enable log feature when using --log-filename

No reason to not assume that the user wants to enable log when the
--log-filename is used. This way uses can skip the use of --log to
enable log.

src/options.c

@@ -372,6 +372,7 @@ void options_parse(int argc, char *argv[])
                 break;
 
             case OPT_LOG_FILE:
+                option.log = true;
                 option.log_filename = optarg;
                 break;