text: Title: Poof() should return ax
  Body: Make sure that ll visualizers return the `self.ax` when calling poof(). This is so that users can always get access to the ax, for example when working inside a function in a notebook. In addition it will encourage the behavior pattern of tweaking the plot if desired after poof(). We will be adjusting our documentation to use this form to also encourage this behavior:
  
  ```
  ax = viz.poof()
  ```
  
  This why this behavior pattern is useful:
  https://stackoverflow.com/questions/47450804/yellowbrick-increasing-font-size-on-yellowbrick-generated-charts
  