scales 0.5.0

• New function regular_minor_breaks() calculates minor breaks as a property
  of the transformation.
• Adds viridis_pal() for creating palettes with color maps from the
  viridisLite package.
• Switched from reference classes to R6.
• rescale() and rescale_mid() are now S3 generics, and work with numeric,
  Date, POSIXct, POSIXlt and bit64::integer64 objects.