Refactors docs on `options.sanitize` in `readme.md`
Refactors docs on `options.sanitize` in `readme.md`

Closes GH-35.

Reviewed-by: Titus Wormer <5a19dbfdb793ef6d212461b3c03b23ad19ae3ed1@gmail.com>
wooorm/mdast-html,wooorm/remark-html,wooorm/remark-html,wooorm/mdast-html