Added ability to override pieces image

[reezpatel]

Added ability to add custom images using pieceTheme property, which accepts a string.

Usage:

const board = Chessboard2('chess-board', {
        pieceTheme: `/assests/chess/{piece}.svg`,
});

Note: I haven't used Clojurescript before, did my best to come up with the solution.

[ernes]

Please @oakmac , it would be great if you merge this. Thanks!

[ernes]

Couldn't this actually close #26 ?

[SC0d3r]

I tried this, and this works can we merge this