add interaction term to rdd function

[darrelrobinson]

Thanks for the package.

A request for a minor change, I propose adding an argument to the RDD function to allow for an interaction term between the treatment and running variable.

[econbernardo]

I believe the interaction term needs to be in the RDD specification, unless one makes additional assumptions (e.g., that the conditional expectation functions of the outcome variable above and below the threshold have the same slope). See, for instance, Chapter 21 in Hansen's Econometrics (2020).