Bump t_Co from 8 to 16 where safe to do so

Doing this means color schemes can do bright colors without forcing
bold.  Do any other terminals not really support at least 16 colors?

plugin/sensible.vim

@@ -51,6 +51,10 @@ if exists('+undofile')
   set undofile
 endif
 
+if &t_Co == 8 && $TERM !~# '^linux'
+  set t_Co=16
+endif
+
 if !exists('g:loaded_matchit') && findfile('plugin/matchit.vim', &rtp) ==# ''
   runtime! macros/matchit.vim
 endif