change open_screen return a screen

hi, change branch version-1.0 ( https://github.com/clearsightstudio/ProMotion/pull/178 )

I came up with this idea when I was using modal view.

before:
```ruby
def action_open_modal
  @modal_screen = ModalScreen.new
  open @modal_screen, moda: true
end
```

after:
```ruby
def action_open_modal
  @modal_screen = open ModalScreen, moda: true
end
```