New plugin: loop (#1989)

* New plugin: loop

Add a plugin that detects loops. It does this by sending an unique query
to our selves. If we see the query more than twice we stop the process.
If there isn't a loop, the plugin disables it self and becomes a noop
plugin.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/loop/loop_test.go

@@ -0,0 +1,11 @@
+package loop
+
+import "testing"
+
+func TestLoop(t *testing.T) {
+	l := New(".")
+	l.inc()
+	if l.seen() != 1 {
+		t.Errorf("Failed to inc loop, expected %d, got %d", 1, l.seen())
+	}
+}