gghighlight 0.3.2

• gghighlight() now accepts n() so that you can highlight based on the
  number of rows within each group (#154).
• gghighlight() now preserves colour or fill when explicit NULL is
  specified on unhighlighted_params (i.e. unhighlighted_params = list(colour = NULL))
  (#152).
• Fix a longstanding bug of wrong calculation when the data has the same name of
  columns as aesthetics, e.g. x (#168).