Add fractals directory, and sierpinski triangle drawer

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Fractals are often computed recursively, with this pull-request, I added a program that displays Sierpinski's Triangle using a given number of iterations (you can test it with 0, 1, 2 or 3, change the width-height if you want to test more).

This was my original solution to a Hackerrank challenge, the code is mine, not sure if having a fractals directory is a good idea, I think it is, would be interesting to see implementations of a mandelbrot visualizer or whatever.