sortableFields should have default sorting options

**Is your feature request related to a problem? Please describe.**

Whenever I log into Netlify CMS, I have to always do the same operation so that the entries of a folder collection is displayed the way I'd like.

**Describe the solution you'd like**

I'd like if we could have an option in the `sortableFields` config so that we could specify a default order and display type. So for instance we could specify something like:

```yml
sortableFields:
display: "grid"
key: "commit_date"
type: "desc"
fields:
- "commit_date"
- "author"
- "title"
````