Support MD ref. link style

[jorgeorpinel]

E.g.

[this is a link][link]
or [even shorter]

[link]: /relative/link
[even shorter]:
  http://full.url/link/with/path#and-anchor?why=not

• Revive/continue guide: cleanup some md links dvc.org#2534 when/if this is implemented.

[jorgeorpinel]

Currently for .md(x) files the following regex is used: /\[.*?\]\((?:<((?:\(.*?\)|.)*?)>|((?:\(.*?\)|.)*?))(?: ["'].*?["'])?\)/gm. This targets [link text](/url/or/path) style links.

Instead of complicating the regex to also match ref. links it's probably easier to do another regex match pass for something like /\[.*?\]\:\s*\n?\s*(URL_REGEX_HERE)/gm. That should match [link id]: /url/path style links.

BTW I'd extract the URL_REGEX in both cases for readability.

[jorgeorpinel]

Please see https://github.com/iterative/link-check/pull/6/files for an initial proposal.