Making the name of the S3 backup directory configurable

When the Mongo backups are uploaded to S3, they're stored under the "backups" directory in the configured S3 bucket (i.e. the path to each backup is BUCKET/backups/YYYY-MM-DD_hh-mm-ss.gz).

I updated the module to allow you to configure the name of the directory in S3, so you can set it to a value other than "backups."

The directory name is configurable through any of the following environment variables:
* S3_BACKUP_DIR
* S3_BACKUP_DIRNAME
* S3_BACKUP_DIR_NAME

The MAX_BACKUPS setting will respect the configured directory name, so it will only remove backups that match the given directory prefix.

My use case is that I have a single S3 bucket with hourly, daily, and monthly backups, and I wanted the directory names to reflect which type of backup was in each directory. I also have a different MAX_BACKUPS value for each type of backup.