Add support for configuration

[JansthcirlU]

Example config usage:

• Markdown formatting
• default renderer
• markdownAutoWrap: false

No need to implement the full schema, but here's the latest version of config.schema.yaml anyway.

Note that the config can be combined with the flowchart title like this:

---
title: this is the title
config:
    flowchart:
        defaultRenderer: "elk"
---

Followed by the flowchart definition to show both the title and render using the elk engine.