labelled 2.7.0

• a .values argument has been added to set_na_values() and set_na_range(), allowing to pass a list of values
• a .strict option has been added to set_variable_labels(),
  set_value_labels(), add_value_labels(), remove_value_labels(),
  set_na_values() and set_na_range(),
  allowing to pass values for columns not observed in the data (it could be useful
  for using a same list of labels for several data.frame sharing some
  variables) (#70)
• copy_labels() is less restrictive for non labelled vectors, copying variable label
  even if the two vectors are not of the same type (#71)
• a .strict option has been added to copy_labels() (#71)