6.0.0

• d4a830b Update unist-util-visit
  (breaking, potentially, if you or your dependents use typescript)
• 2f56170 Add tabIndex: -1 to hidden links
  (breaking if you want links to be hidden but still tabable, which is most likely not good for accessibility)
• c5b3392 Add support for grouping heading with link
• e8a15b8 Add support for before, after options
• 56dbca9 Add support for a content function
• b0dc98b Refactor prose