Roadmap: Change codeblock syntax

[Aetherinox]

Summary

Gist codeblock syntax to be modified and move away from an individual URL to a codeblock that supports property names and values.

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Tasks

• Add new regex pattern with matching groups
• Revise methods to utilize new pattern
• Allow for regular URL without properties / values

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Example

```gistr
url:          https://path/to/username/gist_id
background:   https://path/to/image.png
theme:        dark
```