input: Write a python function to split a string using a specified delimiter.
  assert split_string('This is a sample string', ' ') == ['This', 'is', 'a', 'sample', 'string']
  assert split_string('apple,banana,orange', ',') == ['apple', 'banana', 'orange']
  assert split_string('one-two-three-four', '-') == ['one', 'two', 'three', 'four']
output: def split_string(s, delimiter):
      return s.split(delimiter)