Adding powershell to chef shell-init

This partially replaces https://github.com/opscode/chef-dk/pull/187.
This needs to be placed on top of https://github.com/opscode/chef-dk/pull/254
:construction:

cc @btm @adamedx @opscode/client-engineers 
You need to add `posh` here too - this message got updated when `SUPPORTED_SHELLS` did.