feature request: SFTP support

[petersilva]

It would be great to provide an sftp://user@server/... /file.zsync and have it download using sftp, rather than https.
sftp supports seek(), so it is doable in that sense of being able to update remote files piecemeal.

Ideally, it would be great to be able to use it for "push" as well as pull. in the sense that you go get a remote .zsync file, and look at the local file, and use that the send the different bits over an sftp connection.