Custom delimiter for list widget

**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