Better transparent terminal support

Sets the background color in cterm and gui to 0 and #000000 when
g:zenburn_transparent is 1.

Signed-off-by: Jani Nurminen <slinky@iki.fi>

colors/zenburn.vim

@@ -63,10 +63,11 @@
 " list of configurable parameters.
 "
 " * You can now set a darker background for bright environments. To activate, use:
-"   contrast Zenburn, use:
-"
 "      let g:zenburn_high_Contrast = 1
 "
+" * For transparent terminals set the background to black with:
+"      let g:zenburn_transparent = 1
+"
 " * For example, Vim help files uses the Ignore-group for the pipes in tags
 "   like "|somelink.txt|". By default, the pipes are not visible, as they
 "   map to Ignore group. If you wish to enable coloring of the Ignore group,
@@ -152,6 +153,10 @@ if ! exists("g:zenburn_high_Contrast")
     let g:zenburn_high_Contrast = 0
 endif
 
+if ! exists("g:zenburn_transparent")
+    let g:zenburn_transparent = 0
+endif
+
 if ! exists("g:zenburn_color_also_Ignore")
     let g:zenburn_color_also_Ignore = 0
 endif
@@ -452,6 +457,18 @@ else
     hi ColorColumn     guibg=#484848
 endif
 
+if exists("g:zenburn_transparent") && g:zenburn_transparent
+    hi Normal             ctermbg=0     guibg=#000000
+    hi Statement          ctermbg=NONE
+    hi Title              ctermbg=NONE
+    hi Todo               ctermbg=NONE
+    hi Underlined         ctermbg=NONE
+    hi DiffAdd            ctermbg=NONE
+    hi DiffText           ctermbg=NONE
+    hi ErrorMsg           ctermbg=NONE
+    hi LineNr             ctermbg=NONE
+endif
+
 if exists("g:zenburn_old_Visual") && g:zenburn_old_Visual
     if exists("g:zenburn_alternate_Visual") && g:zenburn_alternate_Visual
         " Visual with more contrast, thanks to Steve Hall & Cream posse
@@ -474,7 +491,11 @@ else
         hi Visual        guibg=#0f0f0f
         hi VisualNos     guibg=#0f0f0f
         if &t_Co > 255
-            hi Visual ctermbg=0
+            if exists("g:zenburn_transparent") && g:zenburn_transparent
+                hi Visual ctermbg=235
+            else
+                hi Visual ctermbg=0
+            endif
         endif
     else
         " low contrast