input: Write a python function to extract values from a nested dictionary given a set of keys.
  assert extract_values(config, ['templates', 'index'])=='templates/index.html'
  assert extract_values(config, ['static', 'css'])=='static/css/main.css'
  assert extract_values(config, ['credentials', 'db_host'])=='localhost'
output: def extract_values(nested_dict, keys):
      for key in keys:
          nested_dict = nested_dict[key]
      return nested_dict