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.

まず issue #12 と pull request #20 の古い PR ディスカッションを読み、特に issue にリンクされているコメントを確認してください。最終的な Node モデルと、inline links、autolinks、reference links をどのように区別すべきかを明確にしてください。link reference definitions と source link types が round-trip rendering のために保持されれば完了です。