Add icon argument to scaffold post-type command

Right now no `menu_icon` is set during the scaffolding of `post_types`. 

I implemented a new optional argument for `$ wp scaffold post-type` to give developers the option to add an icon from the official [dashicons](https://developer.wordpress.org/resource/dashicons/) in a very easy way. 

I also implemented a default value that is equal to the standard value so the icon argument is optional. But by adding the extra line in the generated `post_type` code the developer is hinted to the existence of this easily customisable icon. 

Lastly I added some behat tests to test with and without the argument.