Separate plots from computations

[DominiqueMakowski]

Related to #4, having a "structure" for BayesTests would allow separating the plotting aspect: instead of creating a plot and returning it automatically, we could isolate the code and create methods for plot(x::BayesTest{BayesTestCorrelation}).

[tszanalytics]

Yes, the plotting ecosystem seems to be a bit of mess since v1.0 also, so separating them out makes a lot of sense. Unfortunately I have almost no experience with object oriented programming, so I have avoided that up to this point. I think it's a great idea, so if you have a good example I could look at, or want to have a go at a simple version, that would help a lot.