Introduce plot method mediation_model

[cedricbatailler]

Adds a plot method for functions created with mdt_* functions.

library(JSmediation)

data(ho_et_al)
ho_et_al$condition_c <- build_contrast(ho_et_al$condition,
                                       "Low discrimination",
                                       "High discrimination")
mediation_model <-
  mdt_simple(data = ho_et_al,
             IV = condition_c,
             DV = hypodescent,
             M = linkedfate)

plot(mediation_model)