Rule to validate links via https://github.com/tcort/markdown-link-check/

I’ve been using https://github.com/tcort/markdown-link-check/ as a separate check to validate links in markdown. I was starting to see if I could leverage the .markdownlint.json as a config format for that project too and realized maybe it might make sense to just add that functionality as a rule in this project. Because it can go out and create network requests, I think having it off by default would make sense. I may just do it as a custom rule in the end, but I figured I’d check to see if there is interest in this as a core rule.

Example config schema:

    "valid-links": {
      "description": "MD###/valid-links - Validate links",
      "type": [
        "boolean",
        "object"
      ],
      "default": true,
      "properties": {
        "link_types": {
          "description": "Types of links to validate",
          "type": "string",
          "enum": [
            "all",
            "internal",
            "external"
          ],
          "default": "internal"
        },
        "ignore_patterns": {
          "description": "RegExp for ignoring links",
          "type": "array",
          "items": {
            "type": "string"
          },
          "default": []
        },
        "http_headers": {
          "description": "Url and HTTP header pairs when URLs need additional parameters",
          "type": "array",
          "items": {
            "type": "object"
          },
          "default": []
      },
      "additionalProperties": false

/cc @tcort @timmkrause

[borekb]

Just going to note that even a subset of this feature, which would be checking local links like see [below](#another-heading) would be quite useful.

[DavidAnson]

Don’t assign an MDxxx name, just use a name like validate-links and it should be possible to pull it in pretty much as-is. Some other guidance here: https://github.com/DavidAnson/markdownlint/blob/master/doc/CustomRules.md