Added reference to MarkdownIt typings

[m-gallesio]

Added a reference to @types/markdown-it from DefinitelyTyped, replacing the types marked as FIXME and slightly adapting references to Token.children to account for its nullability.

[marijnh]

Thanks!

I'm pretty sure inline and image nodes always have children, so you could use ! there instead of run-time constructs.

[m-gallesio]

Should be done.

[marijnh]

Merged!