Add --model-name option into scaffold_controller generator 

Hi.

I want a feature that

```
$ rails g scaffold_controller Admin::User
```

to be generated.

```ruby
class Admin::UsersController < ApplicationController
  before_action :set_user, only: [:show, :edit, :update, :destroy]

  # GET /users
  def index
    @users = User.all
  end
 ...
```

So I added a --model-name option like this.

```
$ rails g scaffold_controller Admin::User --model-name=User
```

I discussed the feature #11408 .