New features

• New invert_names() function will return a cleaned vector of names
  potentially inverted occurrences of 'Lastname Firstname' to
  'Firstname Lastname' (or the reverse) based on a list of cleaned_names.
  Same as for the expand_names() function, the invert_names(x, x) can be
  used to deduplicate names from a vector without an external source of truth
  (@Bisaloo, #25).