Support loadstreaming/savestreaming FileIO API

[ssfrr]

As of April FileIO.jl has a loadstreaming/savestreaming API for opening files in streaming mode and automatically selecting the decoder/encoder based on the file extension and/or magic bytes.

If this were integrated with TranscodingStreams it would look something like:

using FileIO

loadstreaming("data.txt.gz") do stream
    for line in eachline(stream)
        # do something...
    end
end