Add support for async plugins

This commit adds 2 new components that support
turning markdown into react nodes,
asynchronously.
There are different ways to support async things in React.
Component with hooks only run on the client.
Components yielding promises are not supported on the client.
To support different scenarios and the different ways the future
could develop,
these choices are made explicit to users.
Users can choose whether `MarkdownAsync` or `MarkdownHooks` fits
their use case.

Closes GH-680.
Closes GH-682.
Closes GH-890.
Closes GH-891.

Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>
Reviewed-by: Remco Haszing <remcohaszing@gmail.com>