added ABC Problem

Added two implementations of the ABC problem.
I had the same doubt... The reasons I left these in are:
  1.  In this case the output shows the string that is being found so it's not purely "debugging" info, but more of a "verbose" mode that shows optional but sometimes nice to see info (for instance it shows the string that was successfully built, which could be nice to see when some letter was missing to complete a word)
  2.   I thought that showing how to enable "verbose" debugging output could be valuable to a newcomer... (it certainly wasn't readily available information for me when I was looking for it), but I had the same doubt too.

That said, I have no objection to removing them. As I said I'm not 100% sold on my own arguments, so if you lean on the removal side you can sway me easily! :)