Is your feature request related to a problem? Please describe.
Feature request: define a custom delimiter for the list widget

Describe the solution you'd like
Currently the delimiter , is hard coded.
I'd like to be able to define a custom delimiter e.g. ; since I sometimes need , in my plain text.
, remains as the default delimiter.

Describe alternatives you've considered
Escape , in widget

Additional context
I think this is the relevant file:
https://github.com/netlify/netlify-cms/blob/master/packages/netlify-cms-widget-list/src/ListControl.js#L139