New output channel: Assign custom variables

[Taitava]

Discussed in #127

This output channel assumes the output is a JSON object, reading it's properties and assigning them to custom variables.

For example, consider the following JSON output:

{
    "_link_to_file": "My folder/My note.md",
    "_link_alias": "Meeting notes"
}

This would assign the value My folder/My note.md to a custom variable named {{_link_to_file}}, and Meeting notes to {{_link_alias}}.

TODO:

• Implement the thing.
• Add to CHANGELOG.md
• Write documentation.

[Taitava]

Done. Will be released in 0.22.0.

[Taitava]

This is now released.