Write album, album artist, title, track number, track total, genre, disk number, disk total, and comment tags.
tags:=&mp4tag.Tags{Custom:map[string]string{"CUSTOMFIELD":"custom field","CUSTOMFIELD2":"custom field 2",},Delete:[]string{"genre","cover"},}err:=mp4tag.Write("1.m4a",tags)iferr!=nil{panic(err)}
Write two custom fields named CUSTOMFIELD and CUSTOMFIELD2, delete genre tag, and remove cover.