text: Title: KElbowVisualizer should take a list of k instead of a tuple
  Body: This is just a suggestion that KElbowVisualizer should let the user chose the values of k and not be forced to have a continuous range. 
  This is especially useful when you suppose you have a big number of clusters and you want explore the values of k from 10 to 10 or 100 to 100.
  
  Anyway, thanks for your work.