Refactor for accessible permalinks (fixes #82)

[valeriangalliat]

While this PR is backwards compatible, I deem the changes significant enough to be published as a major version.

This PR is the result of the discussion in Suggestion: adding an option to render a more accessible anchor link (#82).

Browse the readme from this PR.

Overview

Because the default behaviour of permalink: true was inaccessible, and no single solution could satisfy all use cases, I chose to remove the default behaviour and require explicit configuration of a renderPermalink function (which I renamed to just permalink as the boolean option is now obsolete).

For convenience I included a number of different renderers that can be used out of the box, e.g. permalink: anchor.permalink.headerLink() including one with the same markup as the current default anchor.permalink.ariaHidden.

Backwards compatibility

While this will be a major version, I still maintained complete backwards compatibility with the old API. Using the boolean permalink option will still behave like it does today, and if the legacy default renderPermalink function is used (by not explicitly configuring a renderPermalink function), it will use process.emitWarning when first called to inform the user of the new way of handling permalinks.

Ping!

@nhoizey, I'd love to have your input on that implementation, it's very close to what I proposed a couple of months ago in the issue. I know this doesn't bring a consensus on a single implementation like you would have ideally wanted, but I hope you still find this to be a reasonable improvement over what we had before, at least in the way it raises awareness and doesn't encourage an inaccessible markup by default anymore.

@nagaozen, you're a key contributor of markdown-it-anchor, so I'd like to make you aware of those changes before I integrate them. If you have the time to take a quick look to this PR, let me know if you have any concerns or suggestions. :)

[nhoizey]

@valeriangalliat this looks like a really nice improvement, with multiple options and great documentation to help decide which one to use! 👍