input: Write a python function to determine the sentiment of a given text using nltk's SentimentIntensityAnalyzer.
  assert sentiment_analysis('I love sunny days!') == 'positive'
  assert sentiment_analysis('This is the worst day ever.') == 'negative'
  assert sentiment_analysis('I am feeling okay.') == 'neutral'
output: def sentiment_analysis(text):
      from nltk.sentiment.vader import SentimentIntensityAnalyzer
      sia = SentimentIntensityAnalyzer()
      scores = sia.polarity_scores(text)
      if scores['compound'] >= 0.05:
          return 'positive'
      elif scores['compound'] <= -0.05:
          return 'negative'
      else:
          return 'neutral'