Add zopfli (gzip) compressor

[adam-azarchs]

zopfli is gzip-compatible, at least at the command line level. It does not include a decompressor - its binaries delegate to zlib for that - but its compressor is interesting in that it achieves noticeably better compression ratios than gzip --best, at the cost of being horrifically slow, requiring huge amounts of memory, and not supporting streaming. It's a useful option for people who absolutely, positively need the wide compatibility of gzip and expect to have something be read far more often than it's written, e.g. static web assets. The compressed files it produces are also often a bit faster to decompress, depending on your hardware.