text: Title: Add alpha support for other scatter visualizers
  Body: **Describe the solution you'd like**
  In follow up to @peterespinosa's [recent PR](https://github.com/DistrictDataLabs/yellowbrick/pull/554), we should add an `alpha` (translucency) param to the scatter visualizers that don't currently have one.
  
  **Is your feature request related to a problem? Please describe.**
  The following visualizers need an `alpha` parameter:
  
  - PCADecomposition 
  - PredictionError 
  - ResidualsPlot (implements a default alpha) 
  - text.TSNE (implements a default alpha) 
  
  see discussion in #475