De-duplicating slugs

[arantes555]

Hello,

This is a PR that de-duplicates the slugs assigned as hashes of headers, if there are multiple headers with the same title in the same file, as is done by https://github.com/valeriangalliat/markdown-it-anchor and most markdown to HTML converters.

For example, if there are two ### Example headers in the file file.md, the first header gets assigned file.md#example, and the second one gets file.md#example-2 (for markdown-it-anchor < 6, which VuePress still uses) or file.md#example-1 (for markdown-it-anchor >= 6, as well as markdown-toc, github-slugger, and GitHub).

Currently, check-md wrongly considers these -n anchors as broken. This PR fixes it.

The behaviour is configurable with the introduced uniqueSlugStartIndex option.

I took the liberty of setting the default for uniqueSlugStartIndex at 2, like VuePress, but of course this could be changed to only be 2 with the VuePress preset, and 1 otherwise.

I also had to clear the content caches between runs of the .check function, because the cache content depends on the options passed to it, so the tests of the uniqueSlugStartIndex option would not work without clearing it.

Lastly, I updated README.md and index.d.ts files to reflect my changes, and the ignoreFootnotes change (they had not been updated for it).

[arantes555]

@whxaxes any chance of seeing any more activity on this repo ?

[tex0l]

@whxaxes would it be possible to merge and release this?