• Support 24-bit ("truecolor") SGR sequences (38;2;r;g;b and 48;2;r;g;b),
  which Parse::ANSIColor::Tiny identifies as 'r255g0b0' and 'on_r0g10b20'.
  Since there are sixteen million of them, css() can't define a class for
  each, so html() writes them into a style="" attribute even when it is
  generating classes. Bumps the Parse::ANSIColor::Tiny prereq.
  
• Add 'attr_to_style' method to allow subclasses to customize the CSS
  properties used for an attribute.
  
• Emit the properties of a style in sorted order, so that a 'styles'
  entry with more than one CSS property is reproducible.