nberDates returns a matrix with two columns of yyyymmdd dates
  giving the Start and End dates of recessions fixed by the NBER.
  
nberShade is a generic method for shading recession areas
  on the current plot.  The default version calls
  nberDates() to get a matrix of yyyymmdd dates and then passes
  those dates and all other arguments along to ymdShade.
romerLines draws vertical lines on the current plot at the
  "Romer and Romer" dates when monetary policy is said to have become
  contractionary.