Support using SSH private key

[eddielong]

I was trying this extension but there does not appear to be a way to use private key authentication, only password-based auth. Would be very nice to have this feature.

[zxfrank]

As per the ssh2-sftp-client documentation, you can use privateKey: fs.readFileSync('/path/to/key'), // Buffer or string that contains the problem is that you can't put it in the settings file, I've edited the plugin to read the file and send the buffer to sftp.connect

#5