Issue title: Backblaze B2 support
 Issue body: Please consider supporting Backblaze B2 (somewhat similar to Amazon S3).

See documentation:
https://www.backblaze.com/b2/docs/
 Comments: 
 Comment 0: I have never heard of it before, is it widely used?
 Comment 1: 29k Google hits for "backblaze b2" vs 476k for "amazon s3". 

135 repos with "backblaze" in their name on GitHub.

I don't know market share data. 

 Comment 2: That's a useful indication, thanks. I think a PR would be great!
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