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 
if there is a board texture we use the board texture for the background, if there is no board texture, we draw the cells and their color is based on config value `darkCellColor` and `lightCellColor`.
Highlighted squares are always drawn. If we have a board texture, highlight is drawn over the texture, if we don't have a texture, the cells are not filled with original color, but with the highlight color (50% opacity for light squares).