Improve tokenizer

[xhluca]

• Closes #31
• Makes tokenizing queries much faster since we don't need to rebuild the vocab (can just use the existing vocab from the corpus

TODO

• [x] add better examples
  • [x] Streaming via generator, and calling next
  • [x] tokenize in int, ids, strings, tuple
  • [x] show tokenizing corpus -> tokenize queries
• [ ] add tests for BM25.retrieve under each possible condition:
  • [ ] Tokenized namedtuple
  • [ ] Object with ids and vocab
  • [ ] tuple (not Tokenized namedtuple) of ids and vocab
  • [ ] ids: List[List[int]]
  • [ ] strings: List[List[str]]
• [x] add tests for Tokenizer.tokenize, where it generates:
  • [x] Tokenized namedtuple
  • [x] Object with ids and vocab
  • [x] tuple (not Tokenized namedtuple) of ids and vocab
  • [x] ids: List[List[int]]
  • [x] strings: List[List[str]]
• [x] add tests for Tokenizer.streaming_tokenize