knapsack: add exercise

[colinleach]

The linter is going to complain. Too bad, it's a long URL inside a comment, so live with it.

More seriously, I'm not thrilled with the example.R. I had hoped to vectorize this, but each cell update needs access to the cells above and to the left. Hence needing both matrix indices, hence nested loops. If you have a better idea, please change the file and add yourself as an author.

I've put difficulty 5, copying Python, Julia, C#.