9.6.0 release notes

This release features significant new features and a major refactor of our site alias remote communication. If Drush had a marketing department, they would have insisted on the number 10.0. We don't, so 9.6.0 it is. We did our very best to preserve existing APIs - please report any breakage.

Highlights
============
- Now 100% ✅  on Windows. All PRs tested for regressions (via Appveyor).
- Now 100% ✅ for sqlite and postgres databases. Mysql has always been ✅. Our test bots run these tests every night.
- Add --filter to many commands (#3718). You may now filter output from any tabular commands. An example: `drush pm:list --filter=ctools`. [See docs for details and general info about output formats](http://docs.drush.org/en/master/output-formats-filters/).
- Deprecate drush_invoke_process(), backend.inc and context.inc. These are no longer used by Drush core and commandfiles are encouraged to use their replacements like Drush::drush(). These APIs will be removed in Drush 10. This is a HUGE win for the maintainability of Drush.
- Site aliases may now point into a Docker container #3835
- Unish, our test framework, can now run integration tests without a heavy setUp(). Tests are being ported over.
- Use Drupal DB during bootstrap instead of CLI (#3800). This enables you to run all but the sql:* commands without the mysql client. Useful for minimalist Docker environments.


Other Noteable Changes
=======================
- Fix significant bug where pm:security misses required security updates. (#3766)
- Allow Symfony Console commands to be used in php:cli. (#3770). Did you know that Drush can discover and run standard Symfony Console commands?
- Document how to deal with dynamic usernames in a site.yml file. 9fb7658
- Obsolete the unused and unmaintained core:execute command. Use site:ssh instead. (#3803)
- Add ssh.pipefail config and use it in sql:dump --gzip (#3818)
- PGPASSFILE passed via Process not on commandline (#3833)
- Use --uri from commandline over uri in an alias (#3966)