[Feature request] Add highlights for barbar.nvim

[fnune]

base16-vim currently doesn't highlight the barbar.nvim bar. It could!

Describe the solution you'd like
Here are the groups that need to be defined. The color variables don't exist on base16-vim and need to be mapped to our values:

" barbar.vim highlighting
call <sid>hi("BufferCurrent", fg_current, bg_current, "", "")
call <sid>hi("BufferCurrentIndex", fg_special, bg_current, "", "")
call <sid>hi("BufferCurrentMod", fg_modified, bg_current, "", "")
call <sid>hi("BufferCurrentSign", fg_special, bg_current, "", "")
call <sid>hi("BufferCurrentTarget", fg_target, bg_current, "bold", "")
call <sid>hi("BufferVisible", fg_visible, bg_visible, "", "")
call <sid>hi("BufferVisibleIndex", fg_visible, bg_visible, "", "")
call <sid>hi("BufferVisibleMod", fg_modified, bg_visible, "", "")
call <sid>hi("BufferVisibleSign", fg_visible, bg_visible, "", "")
call <sid>hi("BufferVisibleTarget", fg_target, bg_visible, "bold", "")
call <sid>hi("BufferInactive", fg_inactive, bg_inactive, "", "")
call <sid>hi("BufferInactiveIndex", fg_subtle, bg_inactive, "", "")
call <sid>hi("BufferInactiveMod", fg_modified, bg_inactive, "", "")
call <sid>hi("BufferInactiveSign",   fg_subtle, bg_inactive, "", "")
call <sid>hi("BufferInactiveTarget", fg_target, bg_inactive, "bold", "")
call <sid>hi("BufferTabpages", fg_special, bg_inactive, "bold", "")
call <sid>hi("BufferTabpageFill", fg_inactive, bg_inactive, "", "")

Here's an example from doom-one.vim: https://github.com/romgrk/doom-one.vim/blame/29c3a2a3d5cb3ec001ee5aaa95de3033fbfd7a58/colors/doom-one.vim#L189

Describe alternatives you've considered
Use doom-one.vim.

[GordianDziwis]

Airlines tabs begin to annoy me, maybe I will start with barbar soon and add some highlights.