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 
I don't think it matters in this case, but it's worth mentioning that `export FOO=bar` isn't supported by some older shells: in those cases the assignment and export have to be different statements.