afcharts 0.5.1

• Updated the sequential colour palette to add two new shades (darkest-blue and lightest-blue).
  This means that the sequential palette can now be used to represent five categories, such as deprivation quintiles.

• Renamed na_colour argument of scale_colour_continuous_af and scale_fill_continuous_af to na.value. This was necessary to enable use_afcharts to work with ggplot2 v4.0.1. Added na.value argument to scale_colour_discrete_af and scale_fill_discrete_af.

• use_afcharts gains a reset argument to turn off analysis function styling of charts.

• theme_af now uses theme options set by earlier calls of use_afcharts by default. #51

• Set the main colour palettes to categorical. The Scale_*_discrete_af functions now use the 6 colour cateogrical palette by default, rather than the 4 colour.

• Added af_dark_blue, af_orange and af_grey to give easier access to the hex codes of these colours.

• Added arguments axis_text and axis_title to function theme_af to set
  whether to show axis titles and labels.

• Added argument legend_title to function theme_af to set whether to show
  legend titles.

• The legend argument of theme_af now accepts a legend justification e.g. 'top-right', 'left-centre' #47