Add support for 'themed' tiles

[BayMinimum]

trafficstars

First of all, :+1: for awesome project!

One cool aspect of 2048 is derivatives, such as merging elements to get element with higher atomic number. Let's add support for such themes!

[plibither8]

Hi, thank you :blush:!

Adding themes seems like a good idea, but it wouldn't be my priority right now. If you can work on it and create a PR, I'd love to review it :smile:

Thanks for the suggestions!

[BayMinimum]

Alright, I'll work on it!