Add optional Zstandard compression

Zstandard provides much faster compression and decompression than
gzip at any compression ratio. The "zstandard" Python package is
an optional dependency to enable this new feature.

I tried running `ligolw_sqlite` on a recent gstlal coinc database
to convert it to XML. The uncompressed XML file took 32 seconds to
write and was 300M in size. The gzip-compressed XML file took 52
seconds and was 98M. The Zstandard-compressed XML file took 35
seconds and was 87M in size.

In short, Zstandard compression will give comparable file sizes
to gzip but with compression/decompression time that is negligible
compared to LIGO-LW processing itself.
3 jobs for zstandard in 2 minutes and 3 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #736668
build-python27

00:01:32

passed #736669
build-python37

00:01:33

 
  Docs
passed #736670
docs

00:00:30