Add support for writing atID, plID and cmID atoms

[jornane]

Add support for writing the Artist ID, Playlist ID and Composer ID atoms.

Originally, I just copied the code already existing for the Content ID atom (cnID), which worked fine except for Playlist ID. Upon examining a file purchased from iTunes, it seems that this atom is longer, which was not supported by the code, so I added support for that as well.

I'm not sure why plID is different from the others;

These byte sequences were obtained from a purchased file:

00000000  00 00 00 20 70 6c 49 44  00 00 00 18 64 61 74 61  |... plID....data|
00000010  00 00 00 15 00 00 00 00  00 00 00 00 XX XX XX XX  |............XXXX|
00000020

00000000  00 00 00 1c 63 6e 49 44  00 00 00 14 64 61 74 61  |....cnID....data|
00000010  00 00 00 15 00 00 00 00  XX XX XX XX              |........XXXX|
0000001c

Note byte[3], which is 0x20 for plID and 0x1c for cnID.