Expose gzip.Reader.Multistream

[rgmz]

Somewhat related to #405, it turns out that compress/gzip has a feature to handle trailing data without returning an error.

If you would like the ability to read a valid gzip member and specially handle non-gzip data after the member, then you can use the gzip.Reader.Multistream feature to read the file one member at a time.

golang/go#47809

I created a field called DisableMultistream and not Multistream because gzip.Reader.Multistream is enabled by default. The latter would silently disable Multistream for any existing instances of archiver.Gz{}.

Additional Context

trufflesecurity/trufflehog#2934

[mholt]

Thanks! I have the stupidest style nits to pick with this 😅 but then we can get it merged in right away.

[rgmz]

Done!

[mholt]

Cool -- let's try it. Thanks!