labelled 2.9.0

look_for() improvements:

• new function look_for_and_select() (#87)
• look_for() can now search within factor levels and value labels (#104)

improvements for tagged NAs:

• better printing of value labels (#89)
• new functions user_na_to_tagged_na(), tagged_na_to_user_na() and
  tagged_na_to_regular_na()
• new option explicit_tagged_na in to_factor() and to_character()
• new functions unique_tagged_na(), duplicated_tagged_na(),
  order_tagged_na(), sort_tagged_na() (#90, #91)

other improvements:

• new functions is_user_na() and is_regular_na()
• new set of unit tests (#99)
• trying to apply a value label, na_range() or na_values() to a factor
  will now produce an error
• bug fix in foreign_to_labelled() for Stata files (#100)