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/default.lua

@@ -90,6 +90,8 @@ return function()
       completion = {
         border = { '', '', '', '', '', '', '', '' },
         winhighlight = 'Normal:Pmenu,FloatBorder:Pmenu,CursorLine:PmenuSel,Search:None',
+        col_offset = 0,
+        side_padding = 1,
       },
       documentation = {
         max_height = math.floor(WIDE_HEIGHT * (WIDE_HEIGHT / vim.o.lines)),