Add do-syntax for transcoding streams

[jakobnissen]

It would be nice if there was a constructor TranscodingStream{C,S}(predicate ...) which allows for the do-syntax, like such:

matrix = GzipDecompressorStream(open(path))
    # stuff
    return matrix
end

We have that for most Julia IO as far as I know.

[lassepe]

You can already do

open(GzipDecompressorStream, path, "r") do io
    # stuff
    return matrix
end

Or is this not what you intend to do with the proposed syntax?