Check 'termguicolors' instead of 'nvim'

What we're really trying to detect here is whether the highlighting is
being done using GUI colors or terminal color codes. This change
reflects that. Just because it's Neovim doesn't mean it's GUI colors.

Author: jez

syntax/pandoc.vim

@@ -584,7 +584,7 @@ if g:pandoc#syntax#style#emphases == 1
     if !exists('s:hi_tail')
         for s:i in ["fg", "bg"]
             let s:tmp_val = synIDattr(synIDtrans(hlID("String")), s:i)
-            let s:tmp_ui =  has('gui_running') || has('nvim') ? 'gui' : 'cterm'
+            let s:tmp_ui =  has('gui_running') || (has('termguicolors') && &termguicolors) ? 'gui' : 'cterm'
             if !empty(s:tmp_val) && s:tmp_val != -1
                 exe 'let s:'.s:i . ' = "'.s:tmp_ui.s:i.'='.s:tmp_val.'"'
             else