Add support of padding

[n10v]

It is OPTIONAL to include padding after the final frame (at the end
of the ID3 tag), making the size of all the frames together smaller
than the size given in the tag header. A possible purpose of this
padding is to allow for adding a few additional frames or enlarge
existing frames within the tag without having to rewrite the entire
file. The value of the padding bytes must be $00. A tag MUST NOT have
any padding between the frames or between the tag header and the
frames. Furthermore it MUST NOT have any padding when a tag footer is
added to the tag.

http://id3.org/id3v2.4.0-structure

[n10v]

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