Adding powershell to chef shell-init

This partially replaces https://github.com/opscode/chef-dk/pull/187.
* chef shell-init supports powershell: `chef shell-init powershell`
* posh is aliased to powershell: `chef shell-init posh`

cc @btm @adamedx @opscode/client-engineers 
only issue is we've never tried it, so we don't know if it works with chef-shell init. So I'd rather leave it out until we've done that (and maybe added tests to make sure it keeps working).