Add a YAML FrontMatter metadata parser

[alfredbaudisch]

• Straightforward now that this is implemented: Add support for custom Parsers of Markdown metadata/attributes (for YAML, Joplin, etc); Make the current Elixir Map parser one of the parser options #30
• YAML lib: https://github.com/KamilLelonek/yaml-elixir
• Current parsers: pardall_markdown/metadata_parser

Proposed Solution

1. Parse YAML to Elixir, getting an Elixir map from the YAML FrontMatter string.
2. Then, feed the resulting elixir map to the current ElixirMap parser, this way, there's no need to worry about the low level details

As an example, the JoplinNote parser also uses the ElixirMap parser. This way you don't need to worry about the inner details of what the YAML data contains, the validation is all done by ElixirMap and FileParser anyway.

Example markdown file

---
title: Elixir!
---

Content...

[alfredbaudisch]

@rockchalkwushock Wanna tackle this one? I added the steps to the issue.

[rockchalkwushock]

@alfredbaudisch yep I think I can tackle this pretty easily. I will review it more this afternoon and if I have any questions post them here.