Allow themes to specify the bold/italic accents.

If the pallette's accents dictionary already has bold/italic values,
don't overwrite them.
Respect Solarized bold/italic global settings.

The Solarized color scheme has two global variables g:solarized_bold and
g:solarized_italic which allow you to completely disable bold or italic
in the colorscheme. The solarized theme should respect those values for
consistency.