text: Title: Add legend option to PCA visualizer 
  Body: The ``PCADecomposition`` allows us to set the color of the different entries. It would be cool to add a legend to describe the coloring.
  
  ### Proposal/Issue
  
  The ``DataVisualizer`` class supports creating legends, but it requires that ``y`` is passed into ``.fit``. Admittedly, PCA doesn't require labels, but sometimes you have them. I would love to label the colorings in PCA. Could the constructor accept a ``legend`` parameter that is similar to the ``color`` parameter, but accepts labels?
  
  