input: Write a python function to filter a list of words based on a given input string, ignoring case.
  assert filter_words(['react', 'javascript', 'node', 'python', 'java'], 'java')==['javascript', 'java']
  assert filter_words(['react', 'javascript', 'node', 'python', 'java'], 'py')==['python']
  assert filter_words(['react', 'javascript', 'node', 'python', 'java'], 'react')==['react']
output: def filter_words(word_list, input_str):
      input_str_lower = input_str.lower()
      return [word for word in word_list if input_str_lower in word.lower()]