Reinstate g:solarized_use16.

Neovim ignores `&t_Co` and `&term`, so it is not possible in Neovim to
use the ANSI colors when `termguicolors` is not set.

Re-add this option to override the value of `s:t_Co`.

Fixes https://github.com/lifepillar/vim-solarized8/issues/71.

templates/_global.colortemplate

@@ -1,4 +1,5 @@
 # vim: ft=colortemplate fdm=marker
+#let s:t_Co = get(g:, '@optionprefix_use16', 0) ? 16 : s:t_Co
 #let s:italics = s:italics && get(g:, '@optionprefix_italics', 1)
 #let s:termtrans = get(g:, '@optionprefix_termtrans', 0) && !has('gui_running')
 Include: _linked_groups.colortemplate