Add knapsack exercise

[JohnMertz]

This is my first crack at trying to contribute an exercise, so please feel free to point me in the right direction if I've done something incorrectly. I read the docs and tried to make the commit as similar as possible to other exercise PRs.

If all goes well, I'll be happy to create more, for want of having any more to do! :)

The discussion is located here.

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[m-dango]

I don't believe the empty cpanfile is necessary, but other than that looks good to me!

[kotp]

Thank you @JohnMertz . I removed the CPANFILE that was empty and would have been the only exercise with it. In case you notice it is gone!

[JohnMertz]

Thanks! The CPANFILE was an accidental confusion. It appears to get generated by the exercise-gen.pl script (or maybe this is just a consequence of my plenv configuration). I'll make sure to omit this in future exercise commits.