Add a Silhouette coefficient plot for `clusterer` module

[tirthajyoti]

Hi,

It would be great to have a Silhouette coefficient plot method to add to the clusterer module.
For many situations, it gives a better visual representation of the number of clusters.

Check my article on Medium on this.

Clustering metrics better than the elbow-method

[reiinakano]

This is in the metrics module :)

[tirthajyoti]

I did not mean the Silhouette analysis plot. I meant a plot where you don't need to know cluster labels. It plots the Silhouette coefficient to determine the optimal number of clusters when you don't know what that is. Much like the elbow plot.