Run command before connect

[yiiman-dev]

Hi First at all, thanks for excelent project. Some connections on some servers neede execute a few steps for opening port. For example, we have closed our ssh ports on servers, and when we want to connect to that, we need knock port on server to open port, then connect ssh. In another scenario, we have a script for call an api to authenticate with alternate SSO method, then this script will open port for few mins. If you create a optional pre connect command for every target, that is very usefull. Thanks alot