Improvement: Use more unique images as tiles

[vineeththomasalex]

Currently, the same image is used for many tiles. Even in the GameOfThrones example, there are several duplicates. Provide a way to increase number of unique tiles.
How can we tweak get_best_fit_tile() to get more unique images?
We could add a tile_usage_score for each tile in tiles directory. Then factor this score into diff, so that a tile that has been overused will get trumped by another tile(which might not be the best fit, but close enough).

[hanzhi713]

You can take a look at my project which considers this
https://github.com/hanzhi713/image-collage-maker

[Pizzanomicon]

Would love to see this implemented as well!

[kpj]

I took a stab at this in #22, feel free to give it a try.