allow user to pass a custom config file

should help fix #46
not sure if we want a confswitch also
add documentation for GANDI_CONFIG env var
enhance ssh command usage

Allow user to add extra arguments, pass them to ssh directly
This answers use case "gandi vm ssh pb@one ps auxw"
add documentation for ssh extra args parsing
Merge pull request #47 from kalou/config

Allow user to pass args to `gandi vm ssh` command