text: Title: Add usage example for fasttext subword embedding in Pre-trained Word Embeddings tutorial
  Body: Currently we have the fastText subword embeddings available, but it's not included in the pre-trained word embeddings tutorial which is often the first tutorial people checks out. It would be great to add example usage of fastText subword embeddings and show how it helps representing out of vocabulary words.
  
  The usage of fastText subword embeddings is to simply specify `load_ngrams` to True when creating [gluonnlp.embedding.FastText](http://gluon-nlp.mxnet.io/master/api/modules/embedding.html#gluonnlp.embedding.FastText).