Folder collection path based on fields

We can specify `path` template for a folder collection - as described in docs [here](https://decapcms.org/docs/collection-folder/) :
Docs says, it supports template syntax similar to `slug` templates
Slug templates allows use of fields

We would like to organise content based on value of certain fields (including relation type fields)

Example : City collection, with a relation field for state
We would like to configure path for cities collection as `path: '{{fields.state.name}}/{{slug}}'`
So that the content would be saved as shown below

```
- state1
--- city1.md
--- city2.md
- state2
--- city3.md
--- city4.md

```

But looks like, folder collection path template doesnt allow use of fields

Start with the collection-folder documentation linked in the issue and trace how folder collection paths currently apply slug templates. Investigate whether field and relation values are available when the path is generated; done means supporting a path such as `{{fields.state.name}}/{{slug}}` and producing the requested state-based folder structure.

Written by the indexing model from the issue text.