sgplot 0.5.0

• use_sgplot() gains a quietly argument to allow messages to be suppressed
  (#67).

• na_colour arguments are deprecated in scale_fill_continuous_sg()/
  scale_colour_continuous_sg() in favour of new argument, na.value
  (#73).

• Fixed bug where use_sgplot() could not handle passing multiple arguments of
  different types to theme_sg() (#63).

• Vignettes are converted to articles. Vignettes are now not available to view
  locally and only via the
  pkgdown site (#66).