text: Title: Change "alpha" parameter to "opacity"
  Body: In working on issue #558, it's becoming clear that there is potential for naming confusion and collision around our use of `alpha` to mean the opacity/translucency (e.g.  in the matplotlib sense), since in scikit-learn, `alpha` is often used to reference other things, such as the regularization hyperparameter, e.g.
  
  ```
  oz = ResidualsPlot(Lasso(alpha=0.1), alpha=0.7)
  ```
  
  As such, we should probably change our `alpha` to something like `opacity`. This update will impact a lot of the codebase and docs, and therefore has the potential to be disruptive, so I propose waiting until things are a bit quieter.
  