Add lint for a link reference without a corresponding link definition (or vice versa)

Often (especially when refactoring) links using the definition/reference style will get broken. It would be useful for markdownlint to catch this.

Two examples are:

Link Ref without Def

See [Docs][] for more info.

<!-- Should have a link definition below like:
[docs]: https://example.com
-->

Link Def without Ref

See [New Docs][] for more info.

<!-- None of the link definitions match the link reference above. -->
[docs]: https://example.com

[DavidAnson]

Two new rules will be available in the next release of the library:

• https://github.com/DavidAnson/markdownlint/blob/next/doc/Rules.md#md052
• https://github.com/DavidAnson/markdownlint/blob/next/doc/Rules.md#md053

They will report errors for:

• Link and image references missing a label
• Link/image reference definitions that are unused
• Link/image reference definitions that are duplicated

The latter two scenarios can be automatically fixed by the library (by deleting the unneeded reference definition).

/cc @jasonsnell

[DavidAnson]

@jasonsnell Cool. If there’s anything ya’ll think I should be doing here to make it more compelling (other than adding more rules), please let me know.

For example: editor integration. I happen to think the VS Code integration is pretty good (because I wrote it), but maybe you have a different favorite editor.

https://marketplace.visualstudio.com/items?itemName=DavidAnson.vscode-markdownlint