Add ability to analyse Regression Kink Designs

[drbenvincent]

• Card, D., Lee, D. S., Pei, Z., & Weber, A. (2017). Regression kink design: Theory and practice. In Regression discontinuity designs (Vol. 38, pp. 341-382). Emerald Publishing Limited.
• Card, D., Lee, D. S., Pei, Z., & Weber, A. (2015). Inference on causal effects in a generalized regression kink design. Econometrica, 83(6), 2453-2483.