Implement word-count exercise

[kytrinyx]

This adds the stub for the word-count exercise.
It is missing a reference solution.

To finish up this PR:

• implement reference solution
• add yourself to the authors array in the exercise config

If you want the test suite to look different, let me know and I'll regenerate it.

[lucaferranti]

thanks @kytrinyx for revamping this! I am pretty motivated to get it launched and maintain this, but was driven away by work stuff.
If I could get some help with the setting up the test runner (which I already started getting :) ) I can push forwards the exercises part