Borgmatic

* 17, 39: Support for user-defined hooks before/after backup, or on error.
* 34: Improve clarity of logging spew at high verbosity levels.
* 30: Support for using tilde in source directory path to reference home directory.
* Require "prefix" in retention section when "archive_name_format" is set. This is to avoid
accidental pruning of archives with a different archive name format. For similar reasons, default
"prefix" to "{hostname}-" if not specified.
* Convert main source repository from Mercurial to Git.
* Update dead links to Borg documentation.

* 40: Fix to make /etc/borgmatic/config.yaml optional rather than required when using the default
config paths.

* 29: Add "archive_name_format" to configuration for customizing archive names.
* Fix for traceback when "exclude_from" value is empty in configuration file.
* When pruning, make highest verbosity level list archives kept and pruned.
* Clarification of Python 3 pip usage in documentation.

* 13, 36: Support for Borg --exclude-from, --exclude-caches, and --exclude-if-present options.

* 35: New "extract" consistency check that performs a dry-run extraction of the most recent
archive.

* 18: Added command-line flags for performing a borgmatic run with only pruning, creating, or
checking enabled. This supports use cases like running consistency checks from a different cron
job with a different frequency, or running pruning with a different verbosity level.