Grab stderr instead of stdout. Print this if the command errors.
New config option: ExcludeFile (tarsnap -X).
New action: list-expired - lists backups which will be expired (deleted)
when ExpireBackups is set to true.
Don't expire adhoc backups.
Improve logging.

The default logger outputs to stderr by default. Create a new info
logger for printing to stdout where it makes sense (anything not a
log.Fatal).

Set commands to print stdout and stderr as they're running.
Fix arg processing.
Don't print unexpired backups when running expiry.
Create a new -with-current flag to print them with list-expired.