Enable index files editing in same folder as single pages

[martinjagodic]

Currently, we have to create 2 collections if we want to enable the editing of index and single pages in a folder. We also have to use a filter to differentiate them, as Decap is unaware of the difference. This is a suggestion to join those and allow for a new config of

Add a config option index_files that acts the same as files in a folder collection.

We could use the meta.index_file (now used for nested collections) option to get which files are index. It is worth investigating if a separate option should be used.

Example:

- name: news
  label: News
  label_singular: article
  folder: content/news
  meta: {path: {widget: string, label: 'Path', index_file: '_index'}}
  fields: [
    {name: title, widget: string},
    {name: date, widget: datetime},
    {name: body, widget: markdown},
  ]
  index_fields: [
    {name: title, widget: string},
  ]