This patch adds support for tagging in foobar2000 style.
The foobar2000 is a famous audio player in MS Windows. Some audio player on Linux also supports replaygain information with foobar2000 style.(ex. Quod Libet, cmus) But current mp3gain doesn't support it.
If the command line option "-s i", which is already supported, is given, mp3gain will also wirte/remove foobar2000 style id3v2 tag.