Add a model library name as attribute to explainer

[hbaniecki]

example:

library(mlr)

train <- DALEX::HR[1:100,]
train$fired <- ifelse(train$status == "fired", 1, 0)
train <- train[,-6]

# prepare mlr model
task <- mlr::makeRegrTask(id = "task",
                          data = train,
                          target = "fired")

learner <- mlr::makeLearner("regr.randomForest",
                            par.vals = list(ntree = 300),
                            predict.type = "response")

model <- mlr::train(learner, task)

model$learner$package

[pbiecek]

@maksymiuks it would be great to add this feature for the release fall 2019

[pbiecek]

done in 67dafea