add markdown support

[lafleurdeboum]

Adds markdown support from MDeiml's repo. There was also iyakitang's, but it wasn't updated in 3 years. Also, MDeiml's supports YAML/TOML metadata (through syntax injection I guess/hope) and Github Flavored Markdown, which is pretty widespread.

This is still WIP. I'll test the highlighter soon to check at least basic functionality.

Their README states a block and an inline grammar. Don't know how to use that ATM.

[Dav1dde]

@lafleurdeboum I pushed some changes to the branch, it compiles for me but I didn't test if it actually works as you'd expect.

[lafleurdeboum]

Errr email replies don't support markdown either :( I'm reposting these comments on the web interface.

[lafleurdeboum]

It does compile - and it fails to highlight markdown content. I pushed
a highlighter example to showcase the highlighting failure. Run with

cargo r --example=highlighter -- README.md > README.html

to see that markdown content is passed as is. Run with

cargo r --example=highlighter -- Cargo.toml > Cargo.html

to see that toml contents get highlighted, or with

cargo r --example=highlighter -- examples/highlighter.rs > 
highlighter.html

to see that rust contents also get highlighted.

BTW the example commit should be cherry-pickable for future users.
Also, cherry-picking it in #3 would show that markdown
contents do get highlighted in ikatyang's markdown parser (but if we find out how to make this one work I'd be more inclined to use it too).

[Dav1dde]

I fear this is just a limitation of the highlighter crate.

Thanks for adding the example, that's very useful. I don't have time to dig into this, so I am inclined to just go with #3 and if someone figures out how to use this one, we can always change it out later.

[Dav1dde]

Cherry picked your example into master 👍

I'll close this for now, feel free to re-open if you want to look into why it is broken (though I assume it first needs fixing in the highlighter crate).