Add border-collapse values

Author: jsit

autoload/csscomplete.vim

@@ -215,6 +215,8 @@ function! csscomplete#CompleteCSS(findstart, base)
       let values = border_width_values + length_values
     elseif prop == 'border-color'
       let values = color_values
+    elseif prop == 'border-collapse'
+      let values = ["collapse", "separate"]
     elseif prop == 'border-style'
       let values = border_style_values
     elseif prop == 'border-width'