Require plugin declaration in config to use any plugin

This also makes it so `-test` without a config file tests all available
plugins with the default configuration. This mode is very useful for
getting some initial data out of tivan.

cmd/tivan/tivan.go

@@ -56,7 +56,12 @@ func main() {
 	}
 
 	if *fTest {
-		err = ag.Test()
+		if *fConfig != "" {
+			err = ag.Test()
+		} else {
+			err = ag.TestAllPlugins()
+		}
+
 		if err != nil {
 			log.Fatal(err)
 		}