Implement Median log accuracy and Median Symmetric Accuracy

[Ben-Cox]

Hi I'd like to contribute a couple additional metrics that have recently come to my attention as useful.
Metrics like MAPE penalize over-prediction more heavily than under prediction for things with a lower bound of 0. The development of these metrics are in this paper.

I'd like to try to contribute them through a pull request as my first attempt to contribute to a package. I will go through the resources you've provided for contributing and see if I can figure it out.