add practice exercise game-of-life

[depial]

Adding game-of-life from problem-specifications. This is a basic implementations with a straightforward matrix traversal example.jl. Only two things to note:

• Matrices used instead of nested vectors
• Difficulty set at 2, but I could imagine 3 due to use of matrices

For this exercise, the only salient differences between using nested vectors and matrices is likely just to be how the elements are accessed and/or how they are traversed. I think this would be a gentle introduction to using matrices, if a user is new to them.

[depial]

I think 3 makes sense for the reason mentioned.

Changed to 3. Cheers!