labelled 2.6.0

• look_for() has been redesigned:
  • look_for() now returns a tibble
  • columns with multiple values for each variable are now stored as named lists
  • a print method has been added for a clearer presentation of results in the console
  • use lookfor_to_long_format() to convert results with one row per factor level
    and per value label
  • use convert_list_columns_to_character() to convert list columns to
    simpler character vectors
  • generate_dictionary() is an equivalent of look_for()
• set_variable_labels, set_value_labels, add_value_labels,
  and remove_value_labels now accept "tidy dots" (#67 @psanker)
• new function names_prefixed_by_values() to get the names of a vector
  prefixed by their corresponding value