add topics to clock exercise in config.json

[robphoenix]

addresses #353

As I mentioned in #357 I was tempted to bump the difficulty up here as well, because I found there was a fair bit of research into classes involved, (__repr__, __eq__) even though it may not necessarily be difficult to implement. In the end I felt that having to research a problem doesn't make it more difficult. Again it's that fine line of a problem appearing difficult when we don't know the answer, and then losing some of that appearance of difficulty once solved.

[Dog]

Thanks