single-element character vector indicating the output ID
map to modify (if invoked from a Shiny module, the namespace will be added
automatically)
session
the Shiny session object to which the map belongs; usually the
default value will suffice.
deferUntilFlush
indicates whether actions performed against this
instance should be carried out right away, or whether they should be held
until after the next time all of the outputs are updated.