Allow ColumnGroup to accept an element render (React)

[Wingedillidan]

Unless the typing for v7 (or documentation in v8 for that matter) is not configured correctly, it seems that you can't supply a customized header group element. Only a string can be supplied. Would be really cool to allow for a customization like...

[
  {
    header: <span>Cool Customization</span>
    columns: [...]
  },
  ...
]