This commit adds simple variable expansion using either `$FOO`
or `${FOO}` when evaluating the config file. This can be disabled
by any command by using the `-no-expand-env` flag.
This commits adds support for non-AWS S3-compatible storage such as
MinIO, Backblaze B2, & Google Cloud Storage (GCS). Other backends
should also work but some code has been added to make URL-based
configurations work more easily.
