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 
posh is an alias for powershell. There is a test for it

On Wed Dec 03 2014 at 10:37:24 AM Adam Edwards <notifications@github.com>
wrote:

> In lib/chef-dk/command/shell_init.rb:
>
> >          end
> >          0
> >        end
> > +
> > +      def export(shell, var, val)
> > +        case shell
> > +        when 'sh', 'bash', 'zsh'
> > +          posix_shell_export(var, val)
> > +        when 'powershell', 'posh'
>
> 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).
>
> —
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> <https://github.com/opscode/chef-dk/pull/253/files#r21253524>.
>