feat: add APEv2 tag support for undo functionality

- Implement native APEv2 tag reading/writing (no external dependencies)
- Add -u option to undo gain changes from stored tag info
- Store MP3GAIN_UNDO and MP3GAIN_MINMAX tags when applying gain
- Compatible with original mp3gain APEv2 tags

Closes #2

Author: m_igashi

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mp3rgain"
-version = "0.3.0"
+version = "0.4.0"
 edition = "2021"
 authors = ["Masanari Higashi <M-Igashi@users.noreply.github.com>"]
 description = "Lossless MP3 volume adjustment - a modern mp3gain replacement written in Rust"