Replace `renderers` w/ `components`, remove HTML parser from core (#563)

* Replace `renderers` w/ `components`
* Replace `allowNode` w/ `allowElement`, which is now given a hast element (as
  the first parameter)
* Replace `allowedTypes` w/ `allowedElements`
* Replace `disallowedTypes` w/ `disallowedElements`
* Change signature of `linkTarget` and `transformLinkUri`, which are now given
  hast children (as the second parameter)
* Change signature of `transformImageUri`, which is now given the `alt` string
  as the second parameter (instead of the fourth)
* Replace `plugins` w/ `remarkPlugins` (backwards compatible change)
* Add `rehypePlugins`
* Change `includeNodeIndex` to `includeElementIndex`: it still sets an `index`,
  but that value now represents the number of preceding elements, it also sets a
  `siblingCount` (instead of `parentChildCount`) with the number of sibling
  elements in the parent
* The `columnAlignment` prop is no longer given to table elements: it’s
  available as `style` on `th` and `td` elements instead
* The `spread` prop is no longer given to list elements: it’s already handled

Remove buggy HTML parsers from core

* If you want HTML, add [`rehype-raw`](https://github.com/rehypejs/rehype-raw)
  to `rehypePlugins` and it’ll work without bugs!
* Remove `allowDangerousHtml` (previously called `escapeHtml`) option: pass
  `rehype-raw` in `rehypePlugins` to allow HTML instead
* Remove `with-html.js`, `plugins/html-parser.js` entries from library
* Remove naïve HTML parser too: either use `rehype-raw` to properly support
  HTML, or don’t allow it at all

Closes GH-549.
Closes GH-563.

The following issues are solved as rehype is now available:

Closes GH-522.
Closes GH-465.
Closes GH-427.
Closes GH-384.
Closes GH-356.

The following issues are solved as a proper HTML parser (`rehype-raw`) is now
available:

Closes GH-562.
Closes GH-460.
Closes GH-454.
Closes GH-452.
Closes GH-433.
Closes GH-386.
Closes GH-385.
Closes GH-345.
Closes GH-320.
Closes GH-302.
Closes GH-267.
Closes GH-259.

The following issues are solved as docs are improved:

Closes GH-251.