Handle only bg or fg set when resolving cursor hl

The previous code to resolve the cursor highlight didn't allow for only
the background or foreground to be set, i.e. only one of guifg & guibg.

This was done intentionally to keep the code simple, on the assumption
that only setting either background or foreground was so rare that it
didn't warrant the additional complexity, but it seems that assumption
was misguided, while it is unusual, it's not rare enough to ignore.

See #84 for background

Thanks @casr for reporting this and researching in some detail :-)

Author: mmrwoods

autoload/fuzzyy/utils/popup.vim

@@ -64,19 +64,27 @@ def ResolveCursor()
         gui: { 'reverse': true },
     }
     var attrs = hlget('Cursor', true)->get(0, {})
-    if !attrs->get('guifg') || !attrs->get('guibg')
+    if !attrs->get('guifg') && !attrs->get('guibg')
         hlset([fallback])
         return
     endif
-    var special = ['NONE', 'bg', 'fg', 'background', 'foreground']
-    var guifg = attrs->get('guifg')
-    var guibg = attrs->get('guibg')
+    var special = ['bg', 'fg', 'background', 'foreground']
+    var guifg = attrs->get('guifg', 'NONE')
+    var guibg = attrs->get('guibg', 'NONE')
+    if has('gui')
+        hlset([{name: 'fuzzyyCursor', guifg: guifg, guibg: guibg}])
+        return
+    endif
     if index(special, guifg) != -1 || index(special, guibg) != -1
         hlset([fallback])
         return
     endif
-    var ctermfg = attrs->get('ctermfg', string(colors.TermColor(guifg)))
-    var ctermbg = attrs->get('ctermbg', string(colors.TermColor(guibg)))
+    var ctermfg = attrs->get('ctermfg',
+        guifg ==# 'NONE' ? 'NONE' : string(colors.TermColor(guifg))
+    )
+    var ctermbg = attrs->get('ctermbg',
+        guibg ==# 'NONE' ? 'NONE' : string(colors.TermColor(guibg))
+    )
     try
         hlset([{
             name: 'fuzzyyCursor',