Adding a command line option to `nxc connect` for a custom SSH key.

By default, the nxc connect command runs an SSH command without specifying the key to be used. Therefore, if one has several identity keys, SSH may fail.

I am afraid it is a solution to one's specific problem which many would not face, and a poor approach at it. It consists indeed of adding some text to the SSH command line. Feels poorly integrated.

However, it is key to my workflow. If you do not mind, please accept this modification.