Patch to chef-dk to resolve issue #186

- Add powershell and cmd to chef shell-init
- Handle stray quotes in powershell
- Powershell documentation in help message
Rather than looping, can we created a shared examples group for these behaviors and then create a test context for each shell? The reason for this is if we need to specify different behaviors for different shells in the future, we will already have a place in the test code that sets up the test case prerequisites for that shell, e.g.,:

```ruby
context "on zsh" do

  include_examples "unix shells"

  it "does zsh things"
end

context "on bash" do

  include_examples "unix shells"

  it "does bash things"
end
```

https://www.relishapp.com/rspec/rspec-core/v/3-1/docs/example-groups/shared-examples