Adding an additional size format, highlighted squares

This PR contains the updates described in #3 without any code refactoring.

todo:
 - add new size format (20px - 200px)
 - add highlighted squares
 - alias for theme (piece) and texture (board)

@marcosdsanchez 
@ajessu 
Trying to understand this early return: If there's no board texture, we won't draw a cell, but we would draw a highlighted cell?