Only set the background after entering vim

This allows vim to guess the value of &background. This will set the
best colors based on system configuration. This will especially help
while using 16 colors: see #50

This disables the fix for morhetz#7 temporarily while entering
vim. It should still be possible to switch to and from light and dark
mode afterwards.

colors/gruvbox.vim

@@ -546,10 +546,12 @@ call s:HL('Normal', s:fg1, s:bg0)
 " Correct background (see issue #7):
 " --- Problem with changing between dark and light on 256 color terminal
 " --- https://github.com/morhetz/gruvbox/issues/7
-if s:is_dark
-  set background=dark
-else
-  set background=light
+if v:vim_did_enter
+  if s:is_dark
+    set background=dark
+  else
+    set background=light
+  endif
 endif
 
 if version >= 700