Auto Incrementing Numbers

**Is your feature request related to a problem? Please describe.**
Auto-incrementing number fields. As items are added to a collection the default number value is incremented.

**Describe the solution you'd like**
Number widgets could have an `auto-increment` parameter?

**Describe alternatives you've considered**
If it's possible to implement this as a custom widget, I'd be more than happy to do that but I've been digging through widget props and some source code an am not sure if there's anything I can leverage to accomplish this.

**Additional context**
I'm using netlify-cms to manage a podcast and need episode numbers. I was going to just set them in the rss feed as I iterate over the episodes (just set the index) but this wouldn't be reliable in the case that an episode gets deleted at some point