Add a Gibbs sampler for the Horseshoe prior and a Polya-gamma normal scale-mixture expansion

[zoj613]

This PR adds functions for creating sampler steps for a normal regression under a Horseshoe prior as described in "A Simple Sampler for the Horseshoe Estimator" (Makalic, Enes & Schmidt, Daniel, 2015).
It also implements the Polya-gamma normal scale mixture expansion described in "Data Augmentation for Non-Gaussian Regression Models Using Variance-Mean Mixtures" (Polson, Nicholas G., and James G. Scott, 2013) and the multivariate normal sampler in "Fast Simulation of Hyperplane-Truncated Multivariate Normal Distributions" (Cong, Yulai, Bo Chen, and Mingyuan Zhou, 2017).

[codecov[bot]]

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[zoj613]

@brandonwillard i've squashed the commits and addressed the comments made.