Option to use the gzip format instead of the zlib format.

The current implementation does not allow using `deflate-stream` and `inflate-stream` to read and write in gzip format. Rather than loosen the restrictions on `window-bits` to allow the magic number, I've added an explicit `gzip-format` option to the appropriate functions.

This is of course useful for compressing dynamically generated web content, where for historical reasons gzip is preferable to deflate per se.