Add more information to Link nodes (for round-trip rendering)

For #12, we need to preserve link reference definitions. We also need to be able to distinguish between the different link types (inline links, autolinks, reference links).

There is an old PR which might serve as a starting point: https://github.com/atlassian/commonmark-java/pull/20#issuecomment-142782665

I'm not sure yet what the final Node types should look like. Ideally we'd have an interface that all the links can implement for code that doesn't care about what the source representation was.

If someone is interested in working on this, please comment here first so we can figure out what the end result should look like.

Empieza leyendo issue #12 y la antigua discusión del PR en pull request #20, especialmente el comentario enlazado en el issue. Aclara el modelo final de Node y cómo deben distinguirse los inline links, autolinks y reference links. Se considera terminado cuando las definiciones de referencias de enlaces y los tipos de enlaces de origen se conservan para el renderizado round-trip.