Complete support for relative links in Markdown page #474

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Feature request

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What problem does this feature solve?

<!-- this works -->
[about](./foo.md)

<!-- this doesn't -->
[contact](./contact.vue)

We should support all possible extensions here https://github.com/saberland/saber/blob/fb2832440886cc1ed3a5262fdb7a90cee7aa8125/packages/saber/lib/markdown/link-plugin.js#L28

Extensions can be retrieved from api.transformers