Mirror draft command to current post command

[ashmaroli]

Adds two features to draft command:

• Define front matter defaults for new drafts using jekyll_compose.draft_default_front_matter in _config.yml
• Open new draft in editor based on EDITOR OR JEKYLL_EDITOR ENV vars

[ashmaroli]

Apparently, the CI builds are getting stalled after an "EDITOR" instance gets opened...

[DirtyF]

It works like a charm 👍

Same on my computer for the tests:

• on master script/test does not open default editor
• on this branch nano gets fired and prevent the script to exit.

[DirtyF]

As we don't support collections documents for now, we could use a common key for default front matter, to avoid current config duplication:

post_default_front_matter:
    description:
    image:
    source:
      url:
      title:
      author:
draft_default_front_matter:
    description:
    image:
    source:
      url:
      title:
      author:

That could become:

default_front_matter:
    description:
    image:
    source:
      url:
      title:
      author:

[ashmaroli]

@DirtyF I've fixed the tests and rebased the branch.
Also, I opted for a separate draft_default_front_matter so that users can supply different front matter templates for drafts and posts..

[DirtyF]

@jekyllbot: merge +minor