challenges: Keep random exercises in directory "challenges/"

And the existing one (exercise-anagrams) to this new directory.
challenges/split-text: New challenge added

Description in the README file, and first dummy code implemented.
split-text/main.py: Implement basic code for ideal strings

Where there's nothing tricky.
spit-test/main.py: Add an extra check for too long words
Merge branch 'split-text-challenge'