Add more tags to mp4

1) correct a bug with the mdat atom.
Sometimes, the metadata can be at the end, after the audio
Demo :
   get any music file and strip the metadata
   convert it to m4a with ffmpeg
   analyse it with MusicBrainz Picard
   et voila, you have the metadata at the end.

Example of structure of a m4a created that way;
https://www.dropbox.com/s/602yivdstan7506/atom.txt?dl=0

2) add support for the "----" tag
See http://atomicparsley.sourceforge.net/mpeg-4files.html
It gives access to the tags created by MusicBrainz Picard

mp4.go

@@ -72,6 +72,7 @@ func ReadAtoms(r io.ReadSeeker) (Metadata, error) {
 func (m metadataMP4) readAtoms(r io.ReadSeeker) error {
 	for {
 		var size uint32
+		var subname string
 		err := binary.Read(r, binary.BigEndian, &size)
 		if err != nil {
 			if err == io.EOF {
@@ -101,8 +102,76 @@ func (m metadataMP4) readAtoms(r io.ReadSeeker) error {
 				return err
 			}
 			continue
-		case "mdat": // stop when we get to the data
-			return nil
+		case "mdat": // skip the data, the metadata can be at the end
+			_, err := r.Seek(int64(size-8), os.SEEK_CUR)
+			if err != nil {
+				return err
+			}
+			continue
+		case "----":
+			/* Generic atom.
+			   Should have 3 sub atoms : mean, name and data.
+			   We check that mean=="com.apple.iTunes" and we use the subname as
+			   the name, and move to the data atom if anything goes wrong, we jump
+			   at the end of the "----" atom.  */
+
+			// let's read the mean atom
+			var subsize uint32
+			err := binary.Read(r, binary.BigEndian, &subsize)
+			if err != nil {
+				return err
+			}
+			sub, err := readString(r, 4)
+			if err != nil {
+				return err
+			}
+
+			if sub != "mean" {
+				// Something's wrong. Remove 8 read bytes from the size counter
+				// since "----" is not a known atom name, the whole data will
+				// be skipped
+				size -= 8
+				break
+			}
+
+			mean, err := readBytes(r, int(subsize-8))
+			if err != nil {
+				return err
+			}
+			// Remove the size of the mean atom from the size counter
+			size -= subsize
+
+			if string(mean[4:]) != "com.apple.iTunes" {
+				// Something's wrong, skip this atom
+				break
+			}
+
+			// Let's read the name atom
+			err = binary.Read(r, binary.BigEndian, &subsize)
+			if err != nil {
+				return err
+			}
+			sub, err = readString(r, 4)
+			if err != nil {
+				return err
+			}
+
+			if sub != "name" {
+				// Something's wrong
+				size -= 8
+				break
+			}
+
+			b, err := readBytes(r, int(subsize-8))
+			if err != nil {
+				return err
+			}
+			/* Remove the size of the name atom from the size counter.
+			We should now be at the start of the data subatom and size should
+			be equal to the size of the data atom and its header */
+			size -= subsize
+
+			subname = string(b[4:])
 		}
 
 		b, err := readBytes(r, int(size-8))
@@ -110,8 +179,14 @@ func (m metadataMP4) readAtoms(r io.ReadSeeker) error {
 			return err
 		}
 
+		// Allow all known atoms and the valid "----" atoms
 		_, ok := atoms[name]
-		if !ok {
+		switch {
+		case name == "----" && subname == "":
+			continue
+		case name == "----":
+			name = subname
+		case !ok:
 			continue
 		}