fluster.sh: let users expand decoders list instead of overriding it

fluster.sh

@@ -47,15 +47,23 @@ else
 	exit 3
 fi
 
-DEFAULT_DECODERS=""
-for codec in VP9 H.264 H.265
-do
-	DEFAULT_DECODERS="${DEFAULT_DECODERS} FFmpeg-${codec}-v4l2m2m GStreamer-${codec}-V4L2-Gst1.0"
-done
-
 # Skip the machine
 shift
 
-set -x
-exec ${FLUSTER} r -j 1 -s -so "${SUMMARY}" ${SKIPS} -d ${@:-${DEFAULT_DECODERS}}
+DECODERS=""
+for codec in VP9 H.264 H.265
+do
+	DECODERS="${DECODERS} FFmpeg-${codec}-v4l2m2m GStreamer-${codec}-V4L2"
+done
 
+if [ "$1" = "+" ]
+then
+	shift
+	DECODERS="${DECODERS} $@"
+elif [ -n "$1" ]
+then
+	DECODERS="$@"
+fi
+
+set -x
+exec ${FLUSTER} r -j 1 -s -so "${SUMMARY}" ${SKIPS} -d ${DECODERS}