Only output valid ISO-639-2 language codes

This does not solve #9 completely as invalid data can still be
printed, but if tags are done properly with id3ted itself, e.g.:

id3ted --USLT "$(< lyrics):description text:eng" the.mp3

"eng" should always be printed without extra bytes from the
TByteVector::data() method.
Increased unstable version number
Only output language codes containing exactly 3 alphabetic letters; also for COMM frames