Prepare for marked 5.x

[webketje]

Marked 5.x introduces a lot of almost breaking changes: it replaces most options with separate extension npm packages named marked-* (bummer). See: https://marked.js.org/using_advanced#extensions. Essentially this means @metalsmith/markdown must either:

1. switch away from marked as the default markdown renderer
2. include a lot of extra NPM packages by default just to keep feature parity.
3. not include any extra NPM packages and throw with an informative error.

Anyhow this will require a major release.