Find a good markdown library for parsing: Test pulldown-cmark implementation

[saschagrunert]

We need an alternative to the markdown crate. For this we should have a look which existing library does the parsing very well and create Rust bindings for it. Which library could we use?

[saschagrunert]

• https://github.com/lise-henry/crowbook
• https://crates.io/crates/hoedown

[saschagrunert]

We also should have a look at #50 and #49

[saschagrunert]

We decided to implement a test implementation of pulldown cmark instead of the markdown crate. The reference implementation can be found in the gutenberg crate.