Make Registry.Build take extra interceptors

Make Registry.Build take an optional list of extra interceptors
that are added after the ones specified by the registry.

registry.go

@@ -14,7 +14,9 @@ func (r *Registry) Add(f Factory) {
 }
 
 // Build constructs a single Interceptor from a InterceptorRegistry
-func (r *Registry) Build(id string) (Interceptor, error) {
+// The extra interceptors are added to the chain after the ones specified
+// by the registry.
+func (r *Registry) Build(id string, extra... Interceptor) (Interceptor, error) {
 	if len(r.factories) == 0 {
 		return &NoOp{}, nil
 	}
@@ -29,5 +31,7 @@ func (r *Registry) Build(id string) (Interceptor, error) {
 		interceptors = append(interceptors, i)
 	}
 
+	interceptors = append(interceptors, extra...)
+
 	return NewChain(interceptors), nil
 }