text: Title: Improvements to PosTagVisualizer
  Body: In #768 we rebooted the part-of-speech tag visualizer to make it more practical for NLP machine learning tasks. Here are a few ideas for follow-up features that could be added:
  
  - [x] optional ordering of the plot by tag count (similar to a frequency distribution). _Note: this should NOT be the default behavior since it could prevent users from being able to compare across plots or across corpora_
  - [x] optional stacked barchart for each category/class if `y` is supplied
  - [ ] option for NER tags with entity recognition (or maybe this should be another visualizer with a shared base class?)
  - [x] optional parsing of raw text (only if nltk or spacy is installed)