text: Title: Color document boundaries (or classes) in dispersion plot
  Body: **Describe the solution you'd like**
  I work with text data a lot and am very excited about the new [`DispersionPlots`](http://www.scikit-yb.org/en/latest/api/text/dispersion.html)! As a future feature, it would be very useful to indicate document boundaries on the plot using a high perceptual contrast colormap, or perhaps a qualitative colormap to color the classes (when classes are available).
  
  **Use case**
  Adding document boundaries (or per-class document coloring) to the plot would enable me to visualize things like:
  
   - n-grams that are very common in only a particular subset of the documents
   - n-grams that are very common across all documents
   - n-gram appearance vs. variance in document length
   - n-gram appearance on a per-class basis (if classes are available)
  
  Update 
  ---------
  @lwgray on August 24th, 2018 
  Issue #576 Doesn't completely resolve this issue thus it will remain open.  However, #576 does  add document boundaries to the dispersion plot