support custom render of select all

[drheinheimer]

Version

1.5.1

Here's what went wrong:

I use a custom render for the list items, but now the select all item looks very out of place. Having a custom renderer prop for the select all button would help resolve this. This might also allow for including a "deselect all" button.

[Aidurber]

Great idea!

[Aidurber]

Closed in v1.8.0