Rudimentary ssh-agent support:

[sterin]

This adds minimal support for ssh-agent, addressing issue #35. It passes the agent option from the configuration file up to the ssh2 library.

An example configuration (in addition to other options):

"host": "my.ssh.server",
"port": 22,
"username": "username",
"protocol": "sftp",
"agent": "SSH_AGENT",

where SSH_AGENT is either paegent on Windows, or the contents of the SSH_AUTH_SOCK environment variable on Linux or macOS (see the documentation of ssh2 for details). When using the agent, there is no need to specify the password, passphrase or private key path.