* Replace cheerio with parse5
* Convert to dependency injection for HTML parser
* Add options.domParser to HTML serializer
* Fallback to native DOMParser if present and no option provided
* Error if no DOM parser is available (option or native)
* Update tests to pass parse5 as config option
* Update test so it passes.
Cheerio interprets `<p><hr /></p>` as one `p` node with a child `hr`
node, but both parse5 and native DOMParser interpret it as 3 nodes: a
blank `p` node, `hr` node, and second blank `p` node. Update test
expectation to match new API.
* Remove cheerio-esque compatibility conversion.
* Use `application/xml` in native DOMParser
Using `text/html` causes it to wrap the fragment in html, body, etc
* Change error message to single line.
Was inserting an undesired newline char
* Add documentation for new `domParser` option to html serializer
Also boyscout missing documentation for `defaultBlockType` option
* Rename `domParser` option to `parseHtml`
Rename the option to make it clearer what it does, since it accepts a
function and not a `DOMParser` analogue object.
