ISBN-verifier: implement new exercise

[pheanex]

Has been started, but not finished yet in #887 and #890
ISBN-Verifier Problem description
This issue is for coordination.

[pheanex]

I would drop the ISBN-13 part, since:

• I don't think its more complex than ISBN-10 and therefore is just a boring coding-task
• problem specifications has no test-cases for it

If somebody insists on adding ISBN-13, I'd suggest:

• using the same verification methods for ISBN-10 and ISBN-13 instead of separate methods
• discussing this (and test-cases) with problem-specifications upstream

Opinions welcome

[cmccandless]

I agree with adhering to the problem spec. I'm although the description does mention ISBN-13, there are no tests currently defined, so I am happy to leave it alone and stick to ISBN-10. If ISBN-13 tests are later added, we can create a new issue/PR to implement them here.

[pheanex]

Ok, then. Ready for review 😄