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.

先阅读 issue #12 和 pull request #20 中的旧 PR 讨论，尤其是 issue 中链接的评论。明确最终的 Node 模型，以及应如何区分 inline links、autolinks 和 reference links。完成的标准是：为 round-trip rendering 保留 link reference definitions 和 source link types。