#179 append to tags

[dbonk]

Currently SoX can only replace id3 tags, it would be nice to be able to read in the original comment tag and then append a new line to it.

(note I do not mean append a new tag(comment) to the whole list of tags(comments) I mean to be able to append text to the comment tag (comment))

I would also be nice to be able to customise the TSSE tag.

I found analysis of the code very confusing in looking for the comment tag code where all tags are called comments. I would consider renaming "comments" to "tags" since only one tag is a comment.