text: Title: Improvements to Silhouette Visualizer
  Body: The following improvements to the Silhouette Visualizer are left over from #91: 
  
  *Note to contributors: items in the below checklist don't need to be completed in a single PR; if you see one that catches your eye, feel to pick it off the list!*
  
  - [x] Improve the documentation describing what Silhouette scores are and how to use the visualizer to qualitatively evaluate a clustering solution. 
  - [x] Find a real world example rather than just using make_blobs (note: we also have an example using the Iris dataset; ideally we'd having something a bit more unique to YB that we can add to `yellowbrick.datasets` module - perhaps this should be a separate issue?). 
  - [x] Instead of hard fixing the limits of the X-axis from -1.0 to 1.0; be more flexible so that the visualizer has a better display (or give the user the option of setting the limits). 
  - [x] Move the cluster identity labels away from the middle and to the y-axis.
  - [x] Add ability to define cluster colors and improve color selection methodology. 
  - [x] Add a legend/annotation that describes the average clustering coefficient (e.g. label the red axvline) 