Add support for custom Parsers of Markdown metadata/attributes (for YAML, Joplin, etc); Make the current Elixir Map parser one of the parser options

[alfredbaudisch]

Make it similar to dashbitco/nimble_publisher#16 (thank you @dkuku).

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Original issue was focused on a single new parser for YAML:
For better compatibility with posts from other static website builders.

• https://jekyllrb.com/docs/front-matter/
• Parser: https://github.com/KamilLelonek/yaml-elixir

[dkuku]

I've recently pushed a pr to nimble publisher to have an option for custom parser-do you think this can be done in a way thats compatible between the packages as a hex dependency?
dashbitco/nimble_publisher#16

[alfredbaudisch]

I liked your idea and the approach to dealing with the parsers. Having it as a hex dependency can be useful for PardallMarkdown, so I don't have to re-invent the wheel.

[dkuku]

The Code.eval_string trick may be good if you keep the data inside your app as nimble publisher does. But when you want to allow parsing external folders possibly modified by others it's a security threat.

[alfredbaudisch]

I've recently pushed a pr to nimble publisher to have an option for custom parser-do you think this can be done in a way thats compatible between the packages as a hex dependency?
dashbitco/nimble_publisher#16

@dkuku implemented as per your idea, thank you (a30a748)