Invoke "download to csv" function when clicking on a custom icon in customToolbarSelect

I'm building a data table with your widget, and I'd like to make the user download to csv only the currently selected records, without have to replicate all the code needed to transform data in to CSV and implementing the "onDownload" callback, filtering out not selected rows.

## Expected Behavior
I was expecting some sort of hook or function to invoke, within CustomToolbarSelect callback

## Current Behavior
It seems impossible (by looking at documentation and sourcecode) to invoke the "download to csv" function (handleCSVDownload) when clicking over a custom icon/button put inside the CustomToolbarSelect

## Your Environment

| Tech | Version |
|--------------|---------|
| Material-UI | 4.9.10 |
| MUI-datatables | 2.14.0 |
| React | 16.13.1 |
| browser | Firefox |
| etc | |

The issue names CustomToolbarSelect and handleCSVDownload as the relevant entry points; start by tracing how the custom toolbar callback relates to the existing CSV download behavior. No file or test is identified, so completion should be defined as a documented way to trigger CSV download for only the selected records without reimplementing CSV conversion.

Written by the indexing model from the issue text.