Preliminary support for regression models

[evancofer]

Added preliminary support for regression models (using MatFileSampler) and user-specified evaluation metrics.

[kathyxchen]

Looks good so far! Just add an example of how to pass in metrics via config file (I think you already have this on your to-do list).
I'll update my config file document after we get your PR pushed to master.

[evancofer]

Which tutorial do you recommend adding it to?

[kathyxchen]

Probably in the getting started tutorial. You can pass in the default ROC AUC/AUPRC functions---it would just be there to show that the metrics parameter exists and how easy it would be to pass in some other metrics.

[evancofer]

@kathyxchen This is ready to be merged after you look through it.

[kathyxchen]

LGTM! Thanks for the speedy PR, I'm super excited about having this functionality in Selene. :) The tutorial looks really good. After I update my branch for PR #57, I'll add links to your tutorial in "Getting started" & some other READMEs/docs pages.