Add additional colors to the base16 emacs theme

Known missing colors:

- smerge-mode
  - smerge-refined-removed (diff lines, highlight changed text)
  - smerge-base (lines from "base" for 3-way merges)
  - smerge-lower (diff lines, added background)
  - smerge-upper (diff lines, removed background)
- ansi-color-bright colors (base24)