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 
border is being removed, i'd like to keep the functionality though since we have it, but i need to update the functionality for the 3d pieces. this library will (should) undergo a major refactor in the future, this PR is just to get the functionality requested