Add support for zlib compression

I needed it for something, so.

README.md

@@ -30,14 +30,15 @@ Introducing **Archiver 4.0** - a cross-platform, multi-format archive utility an
 
 ### Supported compression formats
 
-- brotli (br)
-- bzip2 (bz2)
-- flate (zip)
-- gzip (gz)
-- lz4
-- snappy (sz)
-- xz
-- zstandard (zst)
+- brotli (.br)
+- bzip2 (.bz2)
+- flate (.zip)
+- gzip (.gz)
+- lz4 (.lz4)
+- snappy (.sz)
+- xz (.xz)
+- zlib (.zz)
+- zstandard (.zst)
 
 ### Supported archive formats
 

zlib.go

@@ -0,0 +1,52 @@
+package archiver
+
+import (
+	"bytes"
+	"io"
+	"strings"
+
+	"github.com/klauspost/compress/zlib"
+)
+
+func init() {
+	RegisterFormat(Zlib{})
+}
+
+// Zlib facilitates zlib compression.
+type Zlib struct {
+	CompressionLevel int
+}
+
+func (Zlib) Name() string { return ".zz" }
+
+func (zz Zlib) Match(filename string, stream io.Reader) (MatchResult, error) {
+	var mr MatchResult
+
+	// match filename
+	if strings.Contains(strings.ToLower(filename), zz.Name()) {
+		mr.ByName = true
+	}
+
+	// match file header
+	buf, err := readAtMost(stream, len(ZlibHeader))
+	if err != nil {
+		return mr, err
+	}
+	mr.ByStream = bytes.Equal(buf, ZlibHeader)
+
+	return mr, nil
+}
+
+func (zz Zlib) OpenWriter(w io.Writer) (io.WriteCloser, error) {
+	level := zz.CompressionLevel
+	if level == 0 {
+		level = zlib.DefaultCompression
+	}
+	return zlib.NewWriterLevel(w, level)
+}
+
+func (Zlib) OpenReader(r io.Reader) (io.ReadCloser, error) {
+	return zlib.NewReader(r)
+}
+
+var ZlibHeader = []byte{0x78}