fredr 2.1.0

• fredr() / fredr_series_observations() now always return realtime_start
  and realtime_end columns. These are useful when setting the
  realtime_start / realtime_end arguments, or when using the vintage_dates
  argument (#88).

• Fixed a bug in fredr() / fredr_series_observations() so that now the
  vintage_dates argument properly accepts a Date vector of length 1 or
  greater (#89).