Retrieve connect information from ~/.ssh/config

[obreitwi]

In order to reduce duplication, relevant host-specifc configuration should be
optionally extracted from user's openSSH client connection.

This includes proxy configuration, autojump config etc.

[obreitwi]

Update: Basic information (private keys, hostname, agent) is automatically extracted and used. Other features such as jump hosts are still pending because there is no one-to-one equivalent in libssh2 that could be immediately plugged in.