Improve Sum of Multiples exercise

[kytrinyx]

See the discussions in exercism/exercism#2654 and exercism/go#256

There's been a bit of confusion due to the whole "defaults" thing (use 3 and 5 as defaults if no arguments are passed). I made that up sometime at the dawn of Exercism, and I regret it.

I would much rather have a cleaner problem statement with no defaults.

In order to fix this we would need to

• clarify the README
• create a canonical set of shared test inputs/outputs in sum-of-multiples.json
• fix all of the existing implementations