Markdown support

[AnatoleLucet]

At first I thought it was dumb, but after thinking about it ... I think it could be a nice feature for those big documentations.

A little example:

docs/
├── main
│   ├── introduction.md
│   └── conclusion.md
└── main.md

main.md

# My brand new doc!

## Table of contents
- [Introduction](./main/introduction.md)
- [Conclusion](./main/conclusion.md)

---

Here's the kind of imports Destiny should resolve (also works with folders):

• ./something.md
• something.md
• ../something.md

Notice that a markdown file can be linked without specifying a ./. Destiny is currently ignoring imports that are not starting by ./ or ../ because in js / js-like it means it's an import of a module. I think we should work on that so it will be easier to add more languages.