input: Write a function to classify the sentiment of a sentence.
  assert sentiment_classifier("I love this movie!") == 'positive'
  assert sentiment_classifier("I hate this movie!") == 'negative'
  assert sentiment_classifier("This movie is okay.") == 'neutral'
output: def sentiment_classifier(sentence):
      positive_keywords = ['love', 'great', 'fantastic', 'excellent', 'amazing']
      negative_keywords = ['hate', 'terrible', 'awful', 'bad', 'worst']
      
      sentence_lower = sentence.lower()
      
      if any(word in sentence_lower for word in positive_keywords):
          return 'positive'
      elif any(word in sentence_lower for word in negative_keywords):
          return 'negative'
      else:
          return 'neutral'