Introduce slugifyWithState to avoid breaking change with slugify second argument

[valeriangalliat]

#125 is a good suggestion but I'm worried some users pass directly some slugification modules as slugify argument e.g. one could do:

const slugify = require('@sindresorhus/slugify')

const md = require('markdown-it')()
  .use(require('markdown-it-anchor'), { slugify })

And @sindresorhus/slugify takes an optional options object as second argument. Adding state as second argument would break that.

While it's not the way I recommend using it (the readme specifies doing { slugify: s => slugify(s) }), I'd rather avoid risking a breaking change.

So this refactors #125 to 1. pass the whole state instead of state.env so people can do whatever they want with the whole thing, and 2. put this in a slugifyWithState option to avoid a potential breaking change.