WIP: enable zst compression using new compress= kwarg

FIXME: The one-liner expression that many scripts have to pass to
`write_filename` will become unwieldy as the number of available
compression methods grows from 1 to 2 or even more. Instead of this:

```python
write_filename(
	filename,
	compress = compress = 'gz' if (options.output or "stdout").endswith(".gz") else False
)
```

this patch proposes a new `compress_for_filename` helper function:

```python
write_filename(
	filename,
	compress = compress_for_filename(filename)
)
```

However, even this breaks the DRY principle.
Status Job ID Name Coverage
  Build
passed #920385
build-python27

00:00:39

passed #920386
build-python37

00:00:36

 
  Docs
passed #920387
docs

00:00:42