Be able to reference a specific month or day in date field

I have a section for “Events” in this site where I would like the filename to reflect the date of the event (e.g. `2020-05-14-slug.md`), not the date that event page was created.

[In our config](https://github.com/brooklynrail/brooklynrail-platform/blob/master/themes/brooklynrail/static/workflow/config.yml), I have `.date` entered as a field in our events collection.
I also have the path set to
```
path: "{{year}}/{{year}}-{{month}}-{{day}}-{{slug}}"
```
which is adding the current date (today's date) to the filename.

It would be great to be able to add in the specific month or day of the event date to the path with something like `{{fields.date.month}}`.

The relevant code for implementation is located here:
https://github.com/netlify/netlify-cms/blob/21cd3656cfce1d5fcbacd8e6416a9565dae06c5f/packages/netlify-cms-lib-widgets/src/stringTemplate.ts#L94