Include Credible Intervals for A and B metrics in summary

[aaronvgarcia]

I see two options for this addition:

1. include an argument in the summary function such as quantile_probs to be able to modify the quantile function's probabilities when applied to A and B's posteriors.
2. Apply the custom function getCredInt to A and B's posteriors. these values o be printed in an additional section in the summary.

[FrankPortman]

Hi, thanks for your interest. I don't actively maintain this anymore so off the top of my head I'm not sure which one to prefer, but if you submit a PR I will do my best to meet you half-way and see how we can get it landed.