Add Armstrong numbers exercise

[robkeim]

If you'd like to implement this exercise please ensure that you include a test generator for this exercise. Code for the test generators can be found here:
https://github.com/exercism/csharp/tree/master/generators

Problem specification:
https://github.com/exercism/problem-specifications/blob/master/exercises/armstrong-numbers/description.md

Canonical data:
https://github.com/exercism/problem-specifications/blob/master/exercises/armstrong-numbers/canonical-data.json

[robkeim]

I'll take this one