add default title to break_down plot

[pbiecek]

The plot function for break_down objects has no title.

We should:

1. add default title Break Down profile
2. add default subtitle created for the XXX model, where the XXX is extracted from the explainer (explainer$label). (Note that the plot function may take multiple explainers !!! all names shall be added)

Here is an example

library("DALEX")
library("iBreakDown")
titanic <- na.omit(titanic)
titanic_small <- titanic[sample(1:nrow(titanic), 500), c(1,2,6,9)]
model_titanic_glm <- glm(survived == "yes" ~ gender + age + fare,  data = titanic_small, family = "binomial")
explain_titanic_glm <- explain(model_titanic_glm,   data = titanic_small[,-9],   y = titanic_small$survived == "yes")
bd_rf <- break_down(explain_titanic_glm, titanic_small[1, ])
plot(bd_rf, max_features = 3)