Add XM format option

[sapphire-bt]

As suggested in #1, using XM format may lead to better compression due to the format's use of delta-encoding for samples.

[sapphire-bt]

TL;DR: it's probably not worth implementing a .xm file option. See the tables below for some file size comparisons I've done.

File Sizes

MP3 File	Duration	Size (.wav)*	Size (.it)	Size (.xm)	Size (.it.uz)	Size (.xm.uz)
Trance music	06:27	16,764 KB	16,667 KB	16,668 KB	8,331 KB	7,962 KB
Pop music	03:36	9,646 KB	9,305 KB	9,306 KB	4,845 KB	4,739 KB
Rock music	03:56	10,522 KB	10,167 KB	10,167 KB	7,829 KB	7,650 KB
Classical music	02:10	2,837 KB	2,811 KB	2,811 KB	653 KB	651 KB

*8-bit/22 kHz.

Compression Difference

MP3 File	Size (.it.uz)	Size (.xm.uz)	Difference (KB)
Trance music	8,331 KB	7,962 KB	-369 KB	-4.43%
Pop music	4,845 KB	4,739 KB	-106 KB	-2.19%
Rock music	7,829 KB	7,650 KB	-179 KB	-2.28%
Classical music	653 KB	651 KB	-2 KB	-0.32%

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Clearly .xm does compress slightly better than .it, but in my opinion it's not worth the effort of implementing another format just to save a few kilobytes, so I am closing this issue for now.

If anyone feels like adding .xm then pull requests are welcome.

Thanks for the suggestion @peterekepeter.

[peterekepeter]

Wow, thanks for investigating this! Very cool! Some bytes do get shaved off, but yeah the amount of effort might not be worth it.