Different OOS evaluation metrics, user specified

[crahal]

Pseudo R2 is the natural candidate as it applies to probabilities as opposed to categories for binary outcomes, and is nicely interpretable. Ideally a suite of things that work for both binary and continuous DVs are preferable, but we could have a flag to check and catch warn then revert to a default if the specified option isn't amenable.

[dhvalden]

to me: This means Out of Sample Evaluation Metric

[crahal]

Metrics to include:

1. Default to Pseudo-R2
2. Include RMSE
3. Include Cross Entropy Loss.

Include in results object a string which can be used for x-axis label of the out of sample plot (top right).

[crahal]

I've updated code in b97259d to fix the xaxis tick label of subfigure 'c.', but I'd like to leave this open if possible because I think we still need more than two\three metrics choices (I would like to get the IMV into this, for example).