Feature/PR proposal: s3 target support

[nward]

Hi there, I would like to have the ability to store backups on S3.

This is possible already in several ways:

• write locally, sync the dir with an s3 tool, requires lots of local storage
• write to stdout, pipe to an s3 upload tool which works with stdin, works but needs local metadata etc. and does not work with -l auto
• write to an s3 fuse mounted filesystem, which I have had reliability issues with

These options are OK (and I am using the stdout option right now, in fact), but each have drawbacks. Being able to store the entire backup (with metadata) directly on S3 would be great, as would be being able to restore from S3 directly.

I have looked at how this would be implemented, and it looks like we can add an additional target, and perhaps change how target.openfile works to support metadata files.

Would you be open to a PR implementing this functionality?