labelled 2.12.0

New features

• support of variable labels for packed columns, see dedicated vignette (#142)
• new helpers label_attribute(), get_label_attribute() and
  set_label_attribute() to manipulate the "label" attribute on any
  object (#142)
• new functions get_variable_labels(), get_value_labels(), get_na_values()
  and get_na_range() identical to var_label(), val_labels(), na_values()
  and na_range(), respectively
• to_character() method for data frames (#140)