SFTP support

[sourcefrog]

SFTP transport to archives

Uses Rust ssh2 crate

TODO

• [x] Make sftp a conditional feature
• [x] Default username, the same as the local username?
• [x] Mention in the docs
• [x] Doc that only agents are supported for auth, or support more auth methods

Later

• Integration test that either needs real SSH creds, or that sets up an SSH server. (Might be a bit of work.)

Fixes #83