Implement encode middleware (#2)

* Implement encode middleware

* Add missing break; and add missing JSON struct field tag

modules/caddyhttp/encode/zstd/zstd.go

@@ -0,0 +1,23 @@
+package caddystd
+
+import (
+	"github.com/caddyserver/caddy2"
+	"github.com/caddyserver/caddy2/modules/caddyhttp/encode"
+	"github.com/klauspost/compress/zstd"
+)
+
+func init() {
+	caddy2.RegisterModule(caddy2.Module{
+		Name: "http.encoders.zstd",
+		New:  func() interface{} { return new(Zstd) },
+	})
+}
+
+// Zstd can create zstd encoders.
+type Zstd struct{}
+
+// NewEncoder returns a new gzip writer.
+func (z Zstd) NewEncoder() encode.Encoder {
+	writer, _ := zstd.NewWriter(nil)
+	return writer
+}