Filter date for a field entry to use in the slug

I'd like to inject a "Game or Event Year" into the slug. Rather than creating fields for day and year, I'd like to be able to strip the year from the single "Event Date" field entry and use that in my slug.

Basically exactly what you have done for summaries: https://www.netlifycms.org/docs/beta-features/#summary-string-template-transformations.

```
collections:
- label: "Games"
name: "games"
folder: "content/games"
create: true
identifier_field: gamenumber
slug: 'game-{{gamenumber}}-{{opponent.teamname}}-{{gamedate | date('YYYY')}}' <-- something like that
format: 'json'
fields:
- {label: "Game Number", name: "gamenumber", widget: "string"}
- {label: "Game Date", name: "gamedate", widget: "datetime"}
```