Add Completion Window Options (#901)

* Add window.completion.side_padding and window.completion.col_offset

* add col_offset and side_padding options to config/window

lua/cmp/config/window.lua

@@ -6,6 +6,8 @@ window.bordered = function(opts)
     border = opts.border or 'rounded',
     winhighlight = opts.winhighlight or 'Normal:Normal,FloatBorder:Normal,CursorLine:Visual,Search:None',
     zindex = opts.zindex or 1001,
+    col_offset = opts.col_offset or 0,
+    side_padding = opts.side_padding or 1
   }
 end