Add #lang markdown

[LeifAndersen]

No description provided.

[greghendershott]

Thank you very much for the pull request!

I'm reviewing this quickly, not much free time now, so maybe I'm misunderstanding, but:

Wouldn't the new optional argument in-port need to come after the existing optional argument footnote-prefix-symbol? For backward compatibility?

[LeifAndersen]

Yes, you are absolutely correct that adding it to the front breaks backwards compatibility. Although for whatever it's worth, changing it to this causes it to follow the read read-syntax etc. convention. Also, afaik, the functions need to have that api to work in the lang/reader.rkt file. However, if you don't want to break backwards compatibility we could always make other functions we use internally that use this api.

It seems like neither frog nor this library rely on that argument, but I'm not sure of what other code does.

Either way is fine with me though. It's a matter of keeping to conventions and reducing complexity, or keeping backwards compatibility.

[greghendershott]

Pollen uses it.

Actually, I think you simply want to use the existing parse-markdown function instead. It's preferred and will be more efficient with an input-port.

No need to change read-markdown at all; as the comment above it says, it's there only for backward compatibility (ironically :)).