Adding two new styles to mplstyles

1. The first, `538.mplstyle`  is a replication of the plots used on [FiveThirtyEight.com](http://fivethirtyeight.com/), Nate Silver's blog. I wrote an blog article on the replication [here](https://www.dataorigami.net/blogs/fivethirtyeight-mpl) (with some samples). 

2. The second, `bmh.mplstyle`, is a what is used in [Bayesian Methods for Hackers](https://github.com/CamDavidsonPilon/Probabilistic-Programming-and-Bayesian-Methods-for-Hackers). I think it also won the PlotOrNot contest from last year ¯\_(ツ)_/¯?

cc @ChrisBeaumont