Add support for time/date features

[pplonski]

Related to #23

[pplonski]

mljar-supervised supports now date/time features! 🎉 Feature should be translated to Pandas datetime before providing it to AutoML:

df["date"] = pd.to_datetime(df["date"])

Then AutoML will detect datetime features and will transform it by adding numerical features like: year, month, day, weekday, dayofyear, hour, days_diff_to_min (the difference between minimum date treated as reference). The new features will be added if there will be more than 1 unique value. For example, the AutoML will try to construct a year feature, but if all rows will have the same year then such feature will not be added.