String calculator

TDD kata with NUnit and Shouldly. You will need NUnit 3 Test Adapter extension.

Create a method that, given a delimited string, returns the sum of the values.

• empty or null string returns 0
• comma delimited numbers are added together, i.e. "1" returns 1, "1,2" returns 3, etc...
• allow newlines to act as a delimiter, i.e. "1,2\n10" returns 13
• empty lines or values are treated as zeros, i.e. "1,,2\n\n\n5" returns 8
• negative numbers and non-integers throw an exception
• support different delimiters, first line declares the delimiter when started with hash:

  #|
  1|2|10

  returns 13
• delimiters can be any length:

  #--
  1--7--23
  4--4

  returns 39