Step 4 [Add parsing]

* Add test for testAdd3NumbersWithCustomDelimiter - Tests that 3 numbers
separated with with a custom delimiter returns their sum. that validates
"//:\n1;2;3" returns 6;
* The purpose of this test is to validate refactoring, I expect it will
survive refactoring.
* Refactored code to create a tokeniser which will parse the input.