labelled 2.8.0

• new helper recode_if() for recoding values based on condition, variable
  and value labels being preserved (#82)
• look_for() could be time consuming for big data frames. Now, by default,
  only basic details of each variable are computed. You can compute all details
  with details = "full" (#77)
• printing of look_for() results has been updated and do not rely anymore
  on pillar (#85)
• to_labelled() can properly manage factors whose levels are coded as
  "[code] level", as produced by to_factor(levels = "prefixed")
  (#74 @courtiol)
• new function is_prefixed() to check if a factor is prefixed
• bug fix for na_range<- and na_values<- when applied to a data.frame (#80)