Adds header_will_display method to PM::TableScreen

If defined in your subclass, it will call:

`header_will_display`, `header_will_display(view)` or `header_will_display(view, section)`

This is so that you can modify the view of a header before it is displayed. This is commonly done to do things like change the background or text color of the default header view like so:

```ruby
def tableView(tableView, willDisplayHeaderView:view, forSection:section)
  view.tintColor = UIColor.redColor #  Background 
  view.textLabel.setTextColor(UIColor.blueColor) # Text Color
end
```

This new ProMotion way would be:

```ruby
def header_will_display(view) # Don't need the section index in this example
  view.tintColor = UIColor.redColor #  Background 
  view.textLabel.setTextColor(UIColor.blueColor) # Text Color
end
```