constructor: allow nil options

This can make it significantly easier to configure libp2p with optional options.

config/config_test.go

@@ -0,0 +1,23 @@
+package config
+
+import (
+	"testing"
+)
+
+func TestNilOption(t *testing.T) {
+	var cfg Config
+	optsRun := 0
+	opt := func(c *Config) error {
+		optsRun++
+		return nil
+	}
+	if err := cfg.Apply(nil); err != nil {
+		t.Fatal(err)
+	}
+	if err := cfg.Apply(opt, nil, nil, opt, opt, nil); err != nil {
+		t.Fatal(err)
+	}
+	if optsRun != 3 {
+		t.Fatalf("expected to have handled 3 options, handled %d", optsRun)
+	}
+}