title: How to merge spacy Vocab instances?
body: <p>When processing many millions of documents and saving them as spacy documents for later (more processing, visualisation, extracting features), one obvious solution to scaling up is parallel/distributed processing. This means that each parallel process will have their own Vocab that may get extended over time. </p>
  
  <p>When saving documents, clearly one does not want to save the vocab with the document. So to_disk is probably the best option. For loading the document with from_disk, we need a vocab that contains everything from the point in time when the document was processed, or later and from the correct process.</p>
  
  <p>I think it is kind of ugly and clumsy to keep around multiple vocabs and store with each document the id of that vocab. A much better solution would be to create one uber-vocab from the final vocabs from each process after all processes have finished. That vocab could then be used
  with any document processed. </p>
  
  <p>So the question is: is there already a way to merge a bunch of vocabs into one vocab that would contain the union of all of them? </p>