[#1] [E-99] Adding description for problems from 18th to 20th.
[#1] [E-99] Working solution for P18.
[#1] [E-99] Working solution for P19.
[#1] [E-99] Working solution for P20.
[#1] [E-99] P20: Fixing one case more strict than previous version.
Merge pull request #3 from afronski/erlang-learning

Pull request for 99 problems in Erlang implementation