- Styles now use a builder system, to enforce immutability of styles.
- Corrected and cleaned up how style inheritance works.
- Added a brightening function to colours
- HTML formatter will now automatically pick line and highlight colours
if they are not provided in the style. This is done by slightly
darkening or lightening.
Fixes#21.
This is to solve an issue where writers returned by the Formatter
were often stateful, but this fact was not obvious to the API consumer,
and failed in interesting ways.