adding an additional size format, highlighted squares

- the additional format enables setting custom cell size from 20px to
200 px
- added aliases for theme (piece) and texture (board)
updated exception handling
fixed broken tests
added CustomCellSizeValidatorTest
attempt to fix broken test
added IntegerValidatorTest
updated pieces and boards location
deprecating size index format, and removing ConfigLoader
removed config files
added StringOrInteger validation constraint
Generator - added validation for boards and pieces
Generator - don't call drawBorder(), set background transparent
Board - deprecated drawBorder()
updated GeneratorTest
StringOrIntegerValidator - error on empty string
Diagram - updated coordinates
updated DiagramTest
removed BoardTest
updated default config values, and docs
small docs update