bring back the ASCII plots

[avibryant]

They should probably go into rainier-notebook, and be adjusted to be as compatible as possible with the show/density/scatter interface.

Link to original implementation:
https://github.com/stripe/rainier/blob/v0.2.2/rainier-core/src/main/scala/com/stripe/rainier/repl/DensityPlot.scala

[avibryant]

Or, have show pop up a window from a REPL.