Name return values for `Screen.Size` (#487)

This adds names to the return values of the Size function to add
context. This also changes the return declarations to use the shortcut
when both named return values are the same type, by only specifying the
type once.

screen.go

@@ -76,7 +76,7 @@ type Screen interface {
 
 	// Size returns the screen size as width, height.  This changes in
 	// response to a call to Clear or Flush.
-	Size() (int, int)
+	Size() (width, height int)
 
 	// ChannelEvents is an infinite loop that waits for an event and
 	// channels it into the user provided channel ch.  Closing the