Allow for default fields

[budparr]

Following from #140, I'm breaking this issue out.

Allow for default fields. A site owner should be allowed to set up fields in the config file, or elsewhere, which creates consistency and ease of use for content creators:

Here are examples for how others handle this: Prose.io:, Cloudcannon, Siteleaf.

One approach, as mentioned by @benbalter, is that I could have the following in my config:

jekyll_admin:
  default_fields:
    - title
    - description

And then when creating a new post, those fields would be initialized, but empty, by default. This is closest to the prose.io configuration.

[benbalter]

Just to note, if we head down this route, I'd suggest we don't call them "default fields", which could easily be confused with "front matter defaults".

[mertkahyaoglu]

@benbalter Is it possible to group the default front matters under a name like initial_fields for the api so it would be easy to initialize them in the front end. Currently default ones at the root level like this;

[jekyllbot]

This issue has been automatically marked as stale because it has not been commented on for at least two months.

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