More custom holiday

[gabriel-tessier]

Now we handle the case if the user want to replace the holidays datasets with his own, may be better to set a custom datasets that will be merged with the official one so the user can add and overwrite values. (He can already do it by overwrite the datasets that is a dict, but let's make it more clean).
CUSTOM_HOLIDAY_DATASETS
In case of removing a holiday, maybe need to create a NOT_HOLIDAY_DATASETS that will be set in init