These functions can make it easier to to update a style's
contrast when viewed against light and dark backgrounds.
See #353.
Also avoid a segfault when Get is called on a StyleBuilder
that was created using NewStyleBuilder (as opposed to
Style.Builder).
- 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.
