Add BACKUP_KEEP_* options

[fredericoschardong]

This postgres backup image is also based on schickling but stores the backup locally. Nonetheless, a few interesting options are added to improve backup management. They are:

• BACKUP_KEEP_DAYS Number of daily backups to keep before removal. Defaults to 7.
• BACKUP_KEEP_WEEKS Number of weekly backups to keep before removal. Defaults to 4.
• BACKUP_KEEP_MONTHS Number of monthly backups to keep before removal. Defaults to 6.
• BACKUP_KEEP_MINS Number of minutes for last folder backups to keep before removal. Defaults to 1440.

Is this something you could replicate?

[eeshugerman]

We have BACKUP_KEEP_DAYS now, thanks @fulminant! I'm gonna go ahead and close this as I'd rather not complicate things with the extra options. Feel free to reopen @fredericoschardong if you strongly object.