title: How to get word2index from gensim
body: <p>By doc we can use this to read a word2vec model with genism</p>
  
  <pre><code>model = KeyedVectors.load_word2vec_format('word2vec.50d.txt', binary=False)
  </code></pre>
  
  <p>This is an index-to-word mapping, that is, e.g., <code>model.index2word[2]</code>, how to derive an inverted mapping (word-to-index) based on this?</p>