Allow setting the comment field of PassthroughDecoder

[irgendwr]

Hi, the new PassthroughDecoder is very useful, but in my case it's still missing a feature that I require: access to the comment field. My use case is to overwrite the comment header in order to add metadata that my audio player requires.

One way to allow that would be to set the comment field to pub. If this is not desired for future compatibility reasons, my other suggestion would be to add a simple setter function, such as this:

pub fn set_comment(&mut self, comment: Box<[u8]>) {
	self.comment = comment;
}

Any thoughts on this?

Also: thanks for maintaining such a great client/library! ❤️

[roderickvd]

Let's discuss at #791.