Add new theme: base16-256

Bat already has a base16 theme. The new base16-256 theme is for users
of base16-shell, who configure their terminal with a 256-color variant
of a base16 theme. These variants put some of the base16 colors in
elsewhere in the 256-color table to avoid clobbering bright color slots
(ansi codes 8 to 15) with colors that don't respect the ordinary meaning
of that slot (e.g. bright green in ordinary base16 is not green).

For more details, see https://github.com/chriskempson/base16-shell

CHANGELOG.md

@@ -20,6 +20,7 @@
 ## New themes
 
 - Gruvbox, see #1069 (@kyleondy)
+- base16-256 for [base16-shell](https://github.com/chriskempson/base16-shell) users, see #1111 (@mk12)
 
 ## `bat` as a library