add example which registers and uses go custom elements

this example has an assert that checks wether or nor the finalizers ran correctly. If they don't then a mem leak will happen

examples/plugins/registered_elements/internal/custombin/register.go

@@ -0,0 +1,22 @@
+package custombin
+
+import (
+	"github.com/go-gst/go-gst/gst"
+)
+
+// Register needs to be called after gst.Init() to make the gocustombin available in the standard
+// gst element registry. After this call the element can be used like any other gstreamer element
+func Register() bool {
+	return gst.RegisterElement(
+		// no plugin:
+		nil,
+		// The name of the element
+		"gocustombin",
+		// The rank of the element
+		gst.RankNone,
+		// The GoElement implementation for the element
+		&customBin{},
+		// The base subclass this element extends
+		gst.ExtendsBin,
+	)
+}