Auto-discover private key

[mojavelinux]

It would be nice if this library would auto-discover the private key rather than having to read in the file explicitly. I could get behind a boolean flag, a path to a private key, or an environment variable. But having to use the fs API to read the file makes the library very unfriendly for configuration.

[mojavelinux]

gulp-sftp offers this feature, so perhaps that can be used as a starting point.

[mojavelinux]

At the very least, it should be possible to specify the private key as a path.

[mojavelinux]

@zensh You can assign this one to me. I'm partially there in implementing it.