Adds parse_frontmatter option, defaults to false

[neonvoidx]

📃 Summary

This will parse out frontmatter contained in ---. Example:

---
frontmatter
stuff
here
---

📸 Preview

Example markdown slides:

---
id: Slides Test
aliases: []
tags: []
---

#### Advanced Slides Obsidian Plugin 

 https://mszturc.github.io/obsidian-advanced-slides/

---

#### Code
```typescript
const x = () => {
   console.log("HELL WORLD")
}

If parse_frontmatter is set to `true` then first slide rendered in this example would be the Advanced Slides Obsidian Plugin section with link. If false, the frontmatter would be the first slide. 

To note: if you set parse_frontmatter to true, but your slides dont actually have frontmatter this wont work. I can't think of a good way to truly do this, I thought maybe we could see if the content inside the suspected frontmatter is in the format of key value like:

---

key: value

but obviously that's a naive approach and I think this is good enough (for now at least)


## 📝 Checklist

- [x] I ran `make all` locally and checked for errors.
- [x] I have updated the `CHANGELOG.md` file.

[neonvoidx]

Solves #12

[sotte]

Thanks for the PR!